This chapter should be cited as follows:
Tonni G, Grisolia G, et al., Glob Libr Women's Med
ISSN: 1756-2228; DOI 10.3843/GLOWM.419363

The Continuous Textbook of Women’s Medicine Series – Obstetrics Module

Volume 18

Ultrasound in obstetrics

Volume Editors: Professor Caterina M (Katia) Bilardo, Amsterdam UMC, Amsterdam and University of Groningen, Groningen, The Netherlands
Dr Valentina Tsibizova, PREIS International School, Florence, Italy

Chapter

Ultrasound Imaging of Early Embryonic and Fetal Development

First published: September 2023
Updated: August 2026

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INTRODUCTION

Technological advancements of ultrasound systems and the increasing body of knowledge from embryologic studies have enabled a comparison between intrauterine life in vivo and ultrasonographic imaging.^1,2,3 In this chapter, we hope to demonstrate that an overlap between these two disciplines may be possible and that sonographers can now glean further insights into the early periods of human life. Digital imaging and three-dimensional (3D) ultrasound can depict the developing embryo and early fetus in high resolution. The resulting digital datasets have also enabled research institutes to develop and publicly share comprehensive 3D embryonic atlas databases.^4,5,6,7

In the Carnegie system, embryonic development is classified into stages based on the internal and external morphology and degree of organ development observed on optical microscopy.⁸ 

CARNEGIE STAGING

Carnegie stages 15–17

At Carnegie stages 15–17 (Figures 1 and 2, and Video 1), corresponding to a crown–rump length (CRL) in the range of 8–14 mm and a gestational age of 6.5–7 weeks, the rhombencephalon (future fourth ventricle) represents the largest cerebral vesicle located in the superior aspect of the cephalic pole. The rhombencephalon appears as an inverted-pyramid shape halo with a mean length of 3.7 mm and a mean volume of 8.5–20 mm³, with the pons flexure at its apex.^{9,10,11,12,13,14,15,16,17,18,19,20}

Carnegie stages 18–20

At Carnegie stages 18–20 (Figures 3 and 4, and Video 2), the CRL is in the range of 16–21 mm, corresponding to 8–9 weeks’ gestation. The embryo has an axial cephalic diameter measuring 6–8 mm and during this time period the prosencephalic, mesencephalic and rhombencephalic cavities become visible on ultrasound. The mesencephalic cavity, initially located in an anterior position, moves to a superior position through a process known as 'cerebral deflexion'. The cerebral hemispheres, including the choroid plexuses (CPs) of the lateral ventricles (LVs), are not always visible.

When the embryo reaches a CRL of 17 mm (8.2 weeks), the curved mesencephalic cavity (future aqueduct of Sylvius) is located anteriorly with the rostral aspect pointing caudally. The diencephalic cavity (future third ventricle) develops posteriorly, while the mesencephalic cavity moves posteriorly. At this stage, the mesencephalic cavity is wide compared to the total volume of the embryonic–fetal unit. The future foramen of Monro appears as a distinct structure in embryos with a CRL ≥19.5 mm at Carnegie stage 19.^{9,10,11,12,13,14,15,16,17,18,19,20}

Carnegie stages 21–22

At this stage, the embryo has a CRL of 22–26 mm, corresponding to 9 weeks (Figures 5 and 6, and Video 3). The axial cephalic diameter measures 9–12 mm. The CPs of the LVs are visible at this stage. Their volume can be calculated and is in the range of 40–120 mm³, whereas the volume of the LV is in the range of 70–200 mm³. The CPs have a club shape and the cerebral hemispheres have a C-shape with their cortex characterized by a hypoechoic mass. When embryos are ≥ 25 mm, there is a clear difference in the sizes of the rhombencephalic and mesencephalic cavities due to the growth of the cerebellum. The rhombencephalic isthmus is thin and not always visible in its entire length. The mesencephalic cavity is still wide with a distinct prosencephalic isthmus and rhombencephalic isthmus, and the cerebral hemispheres cover three-quarters of the diencephalon.

At this time, the metencephalon (future pons and cerebellum) and myelencephalon (future medulla oblongata) are clearly distinguishable subdivisions of the rhombencephalon).^{9,10,11,12,13,14,15,16,17,18,19,20}

Carnegie stage 23

At Carnegie stage 23, the embryos reach a CRL ≥ 29 mm, corresponding to 10 weeks. The LVs are filled by the CPs, covering the anterior horns, and the diencephalic cavity has a volume >280 mm³. In most cases, the diencephalic cavity is shadowed by the cerebral hemispheres and the falx cerebri starts to develop along the midline. The cerebellar hemispheres become visible, separated along the midline and united at the level of the medullary velum.^{9,10,11,12,13,14,15,16,17,18,19,20}

SONOEMBRYOLOGY

Sixth week of gestation

The embryo at 6 weeks’ gestation measures approximately 5–7 mm, corresponding to Carnegie stage 15. With the advent of real-time high-frequency probes and particularly by using the transvaginal approach, the embryologic structure can be seen within a gestational sac (GS) of 20 mm, measured at the longest longitudinal diameter, and with the accompanying yolk sac (YS). At this stage of development, using 3D ultrasound with HDlive and Silhouette mode, the YS is not seen as a ring structure as when using conventional two-dimensional (2D) ultrasound but as a solid structure adjacent to the living embryo and somewhat resembling a second embryo (Figure 7), although slightly less developed and lacking heart motion. The embryonic heart rate is usually in the range of 90–110 beats per minute (bpm).^{9,10,11,12,13,14,15,16,17,18,19,20}

Seventh week of gestation

This stage is equivalent to Carnegie stage 16 and the living embryo has now reached a CRL of 9–13 mm. The buds of the upper and lower limbs (Figure 8) become visible as well as the relationship between the umbilical cord and the placenta (Figures 9 and 10). At the cephalic pole, the five secondary cerebral vesicles are visible and distinguishable (Figures 11 and 12). These are observed as four anechoic spaces corresponding to the following structures: (i) prosencephalon (forebrain) - this will further develop into two telencephalic vesicles, whose walls will give rise to the cerebral hemispheres and basal nuclei, while their cavities will become the lateral ventricles; (ii) diencephalon - its walls will form the thalamus, hypothalamus and optic vesicles, and its cavity will become the third ventricle; (iii) mesencephalon (midbrain) - from this region, the aqueduct of Sylvius will develop; (iv) rhombencephalon (hindbrain) - this will subdivide into the metencephalon and myelencephalon. The metencephalon will give rise to the pons and cerebellum and the upper part of the fourth ventricle, whereas the myelencephalon will form the medulla oblongata and the lower part of the fourth ventricle. 

The future mid-face is visible as an 'equals' hyperechogenic sign on an axial plane using 2D ultrasound (Figure 10A). An enhanced cephalocervical flexion is another characteristic of the seventh week of gestation. The primordial ophthalmic bud starts to appear in the cephalic pole as an anechoic bulging in the upper aspect, while in the lateral aspect the primordial external ear becomes visible (Figure 13). The heart rate is usually in the range of 130–160 bpm.^{9,10,11,12,13,14,15,16,17,18,19,20}

Eighth week of gestation

During this stage of development, the embryo reaches a CRL of 16–19 mm, corresponding to Carnegie stage 18. The flexion between the cephalic pole and the cervical region becomes evident (Figure 14). The cerebral vesicles are growing, and the cleavage of the prosencephalon can be identified. In a sagittal plane of the cephalic pole, the four cerebral vesicles are seen. The diencephalon, mesencephalon and rhombencephalon are recognizable, as well as the lateral ventricles (Figure 15). Elongation of the fetal trunk is also evident compared with an embryo at 7 weeks. There is progressive reduction of the tail that culminates in tail atrophy. A transverse section at the level of the umbilical cord (UC) demonstrates the umbilical vessels consisting of one umbilical vein (UV) and two umbilical arteries (UAs).^{9,10,11,12,13,14,15,16,17,18,19,20}

Ninth week of gestation

During this week, the embryo elongates, its CRL usually measuring 20–26 mm (Figures 16–18, and Video 4). This corresponds to Carnegie stage 19. The cephalic pole presents 'exaggerated' growth compared with the remaining embryonic anatomy. The developing eyes are now clearly distinguishable, either in a coronal or longitudinal plane. The future nasal bone, primary palate, maxilla and mandible are also detectable. The buds of both upper and lower limbs protrude ventrally, and the fingers become visible at this stage compared with the toes, as they are in a fixed flexed position across the fetal trunk. The trunk has continued its process of elongation and straightens at the abdominal level. The presence of physiological herniation of the mid-gut, appearing as a rounded isoechoic, bulging structure contained within the peritoneal membrane and located at the base of the umbilical cord, is another characteristic finding at this stage of development (Figure 17). This herniation occurs because the growth of the primitive intestine proceeds at a higher rate than that of the abdominal wall. The embryo is approaching the next step of its growth and will finally become a fetus.^{9,10,11,12,13,14,15,16,17,18,19,20}

Tenth week of gestation

During the last phase of embryological development, the embryo measures 22–26 mm, equivalent to Carnegie stages 21–22. The angle between the head and the trunk approaches 30°, with the head representing almost 50% of the total size of the embryo. The calvarium has started its mineralization process; the frontal bone is visible as a hyperechoic contour in the sagittal plane, and the CPs of the LVs are the most developed intracranial structure visible, surrounded by the anechoic cerebrospinal fluid (CSF). The bony structures forming the midface are evident as echogenic lines: the nasal bones, just below the frontal bone; the primary palate, recognizable as a hyperechoic line with an oblique and inner direction; and the hyperechogenic dot of the mandible.^{9,10,11,12,13,14,15,16,17,18,19,20,21}

During this week, physiologic herniation of the mid-gut with its characteristic bulging will normally disappear as the small bowel undergoes a process of re-entering the abdomen. Failure of this process will generate a midline umbilical defect (omphalocele). Right-sided defects (gastroschisis) have a different etiological mechanism.

In humans, six pairs of aortic arches are described, although the fifth arch is usually absent or rudimentary. The first arch is completed by day 24 but begins to remodel by day 26 as the second arch appears. The third and fourth arches are established by day 28, around the time the second arch begins to regress, followed by the appearance of the sixth arch on day 29. While the derivatives of the first three arches remain relatively bilateral, the fourth and sixth arches undergo significant remodeling to develop asymmetrically into the definitive great vessels.²¹ 

The relationship between completed gestational age, Carnegie stage, CRL and the embryonic and early fetal anatomy visible on ultrasound examination is summarized in Table 1.

SKULL, BRAIN AND NECK

With sonoembryology, we have been able to demonstrate an overlap between embryonic development of the early brain and findings on ultrasonography.^{1,2,12,13,14,15,16,17,18,19} Similarly, the skull and midface start their development between 5 and 7 weeks’ gestation, corresponding to Carnegie stages 12–16. The skull is composed of two different portions: the first, called the 'neurocranium', which will house the developing brain and is subdivided into the cranial vault and the cranial base of the skull; the second, called 'splanchnocranium', which will develop into the midface. These bony structures are the most complex bone arrangement of the embryo–fetal skeleton, as they are responsible for protecting the central nervous system (CNS), the eyes, ears, and the oral and nasal cavities. The skull originates from two different and separate mechanisms: intramembranous ossification and endochondral ossification. The formation of these bones follows the general process that involves the cartilage that derives from mesenchymal thickening, i.e. apposition and reabsorption. The neurocranium consists of neural crest cells and paraxial mesoderm that will form the frontal and temporal bones (the cells derived from the neural crest), while those deriving from the paraxial mesoderm will later form the parietal and occipital bones, respectively (Figure 19). The splanchnocranium derives from the first and second pharyngeal arches and is further subdivided into the maxillary process (temporal bones, zygomatic bone, palatine bone, nasal bone, vomer, lacrimal bone and maxilla) and the mandibular process. All the abovementioned bony structures derive from the first pharyngeal arch, whereas the styloid process, the stapes, and the upper part of the hyoid bone originate from the second pharyngeal arch.

The mandible begins to develop around 7–8 weeks' gestation by intramembranous ossification adjacent to Meckel's cartilage and is the first bone of the skull to ossify. The different bones of the skull are interconnected by a series of sutures and six halos of fibrous areas called fontanelles, of which the two most important are the bregmatic fontanelle (the largest and anterior) and the lambdoid fontanelle (the smallest and posterior). As cranial development progresses, ossification of the calvarium begins at around 10 weeks' gestation, first involving the frontal and parietal bones. For this reason, at this early stage of development, ultrasound evaluation and imaging of the embryo-fetal skeleton is scanty, even following the advent of real-time high-frequency equipment and the introduction of 3D ultrasound. However, the use of more recently developed four-dimensional applications allows enhancement in visualization of the cranium.^{22,23,24,25,26}

There are several CNS abnormalities that can be clearly and accurately diagnosed as early as the first trimester of pregnancy, even before 11 weeks’ gestation.^27,28,29 The acrania–exencephaly–anencephaly (AEA) sequence is an embryonic maldevelopmental process that occurs around day 21 of embryogenesis, at the level of the anterior neuropore. A new classification of this sequence, based on specific phenotypic presentations, has been proposed and includes the following patterns: bilobular cranial appearance, cystic, foreshortened, elongated and overhanging types, and more recently the 'turban' sign phenotype (Figure 20). Folic acid metabolism perturbation and caffeine intake are among^30,31 the recognized etiopathogenetic mechanisms, although in some other cases it may be secondary to an early amnion rupture resulting in the creation of amniotic band syndrome with entrapment of cerebrospinal fluid.^{32,33,34,35,36} The presence of an amniotic band may not only be causative of specific AEA phenotypes, as seen for the turban sign phenotype, but may also be responsible for decapitating (decollatio) the developing embryo (Figure 21).

Another defect that is classified as a neural tube defect (NTD) is iniencephaly. This condition, occurring with an incidence of 0.1–10/10,000 births,³⁷ is characterized by hyperextension of the cephalic pole and is usually associated with the presence of spina bifida and other structural defects such as encephalomeningocele, hydrocephalus, Dandy–Walker malformation, cardiac defects, omphalocele and gastroschisis, and renal anomalies.^38,39 Iniencephaly is related to multiple pathogenetic mechanisms, including polymorphisms in the metabolic pathway of MTHFR (methylenetetrahydrofolate reductase) and chromosomal abnormalities.^40,41 The onset of cerebral dysraphism takes place as the first phase of the development of anencephaly and is a mesenchymal defect occurring before days 18–20 and the 28th day post-fertilization, equivalent to Carnegie stages 8–9.^42,43 Iniencephaly diagnosed in the early first trimester is rarely reported in the literature. Tonni et al. have described a case occurring in a 41-year-old mother with polymorphisms involved in folic metabolism (MTHFR, MTRR, CBS), in which iniencephaly was detected at 12 weeks’ gestation in a fetus with proven trisomy 18 on chorionic villus sampling.⁴¹ In a previous observation, iniencephaly was seen associated with alobar holoprosencephaly (HPE) in a fetus with trisomy 13, confirming a potential role of common trisomy and chromosomal abnormalities in the pathogenetic mechanism.⁴⁴

The neurulation process is completed by Carnegie stage 12, corresponding to approximately 5½ weeks of gestation. During this period, the neural plate folds and closes to form the neural tube. About a week later, at Carnegie stages 15–16, the future telencephalic vesicles begin to appear. However, if signaling from the notochord is altered during this stage, abnormal forebrain induction may occur, potentially resulting in alobar holoprosencephaly (HPE).^{45,46,47,48,49,50} (Figure 22)

As noted above, the prosencephalon undergoes a process of cleavage that will give rise to the telencephalon and diencephalon. A subsequent division of the telencephalon forms the cerebral hemispheres and the lateral ventricles, while the thalami and the third ventricle originate from the diencephalon. A complete or partial failure of the telencephalon to separate into two cerebral hemispheres results in a holosphere with a single cerebral ventricle, the characteristic feature of the alobar type of HPE.^46,49,50 This condition has an estimated prevalence of 1 in 250 embryos and is the most common CNS abnormality involving the forebrain.⁵¹ As a gastrulation defect, HPE occurs in a critical period during embryogenesis that ranges from the 18th to the 28th day post-fertilization⁵² and affects 1 : 10,000 live births.^53,54 Although HPE is the consequence of a defective gastrulation process of the neural tube, it can also be seen associated with genetic abnormalities and syndromes and with a defect in the signaling cascade of the Sonic hedgehog (Shh).^55,56,57

Fetuses may exhibit increased nuchal translucency (NT) or cystic hygroma before the 11th week of gestation (Figures 23 and 24). Standard screening for common trisomies is typically performed between 11⁺⁰ and 13⁺⁶ weeks’ gestation, in conjunction with first-trimester biochemical testing and cell-free fetal DNA screening. The distinction between markedly increased NT and early cystic hygroma can be difficult, particularly before the conventional NT screening window.

SKELETAL SYSTEM

The skeletal system as well as the limb buds start to be sonographically visible from 7 weeks. As we have seen, the limb buds start to be detected from week 7 onwards, as well as the fetal spine, especially when using a novel four-dimensional (4D) ultrasound application in skeleton mode.¹⁹ Even at an early stage of development, as early as 10 weeks’ gestation, rare skeletal dysplasias such as radial aplasia with club hand can be diagnosed (Figure 25).

THORAX, ABDOMEN AND URINARY SYSTEM

Body-stalk anomaly is a rare disease that has been reported to have an incidence of 1 in 14,000–42,000 pregnancies, although a study performed in the metropolitan area of London showed a prevalence estimated at 1 in 7500 fetuses at 10–14 weeks’ gestation.⁷³ The anatomical and sonographic diagnostic clusters can vary, but a major anterior abdominal defect with eviscerated organs, kyphoscoliosis, limb defects, and a very short or absent umbilical cord are usually key features of this lethal condition.^72,73,74,75 Although the pathogenesis has not been fully elucidated, a potential mechanism of this embryologic maldevelopment may cause early amnion rupture with subsequent formation of amniotic band syndrome, early embryonic vascular disruption, as well as abnormal embryonic folding occurring at 1–4 weeks postconception.^73,76,77,78 This rare disease is characterized by organ eventration lying in the amniotic cavity, whereas the lower portion of the embryo–fetal body lies in the celomic space (Figure 26). According to Russo et al.,⁷⁹ two different phenotypes can be detected and caused by different pathogenetic mechanisms. One might be secondary to a vascular disruption and characterized by the presence of amniotic band syndrome with craniofacial defect. The other phenotype might be a consequence of embryonic maldevelopment and is associated with urogenital, anal and abnormal abdominal attachment to the placenta secondary to agenesis or severe shortening of the umbilical cord. These anatomical and sonographic defects are diagnostic clues of the condition that should guide the sonographer to an accurate prenatal diagnosis after excluding other pathologic abnormalities of the fetal abdomen, such as omphalocele and/or gastroschisis. This defect is associated with a lethal prognosis.

A right-sided herniation at the level of the umbilical cord insertion invariably results in herniation of the mid-gut, a condition known as gastroschisis (Figure 27). Gastroschisis is not usually associated with chromosomal abnormalities, however, in a series of 24 cases of fetuses with abdominal wall defect (16 omphalocele, eight gastroschisis), 50% of the fetuses with gastroschisis had associated structural anomalies.⁸⁰ Gastroschisis is defined by the presence of a bulky structure at the right side of the umbilical cord that is not covered by a membrane, compared with the finding of omphalocele (Figures 28 and 29),^{81,82,83,84,85,8687,88,89} and not usually associated with fetal ascites. Fetal magnetic resonance imaging (MRI), especially using the ultrafast single-shot spin-echo (SS-SE) technique has been shown to be clinically useful in monitoring the integrity of the loops of small bowel, together with α-fetoprotein level determination by amniocentesis. All these procedures can be integrated antenatally with the intent to capture potential sonographic, radiologic and biochemical indirect signs of bowel injury and to better plan the timing and mode of delivery; elective Cesarean section may facilitate coordinated and prompt surgical intervention by a multispecialist team.^90,91

Lower urinary tract obstruction (LUTO), with an estimated incidence of 0.06%,⁹² can be identified once fetal urine is being produced from 10 weeks’ gestation onwards.⁹³ The prenatal ultrasound diagnosis is usually based on a bladder diameter >7 mm, and >12 mm in severe cases.^92,93,94 A gestational-age-dependent measure suggests the size of the bladder larger than 10% of the CRL.⁹² In a large series of approximately 25,000 pregnancies scanned at 10–14 weeks' gestation, fetuses with megacystis had a minimum longitudinal bladder diameter/CRL ratio of 13%.⁹² In normal fetuses, the bladder was consistently visualized once the CRL reached 67 mm (approximately 13 weeks' gestation). Fetal megacystis can be associated with chromosomal anomalies and for this reason fetal karyotyping is clinically recommended.⁹² Although megacystis is rare,^95,96 it is important to reach a correct prenatal diagnosis, as megacystis may be a feature of other complex congenital malformations such as the omphalocele-exstrophy-imperforate anus-spinal defect (OEIS) complex, in order to provide appropriate prenatal counseling and estimation of recurrence risk, even though in utero and perinatal prognosis is sometimes challenging.^94,97 Two recent surveys have evaluated the performance of ultrasound in detecting fetal congenital anomalies. The first, published in 2019 by Bardi et al.,⁹⁸ reported a detection rate of 100% for megacystis between 11⁺⁰ and 13⁺⁶ weeks’ gestation. In contrast, a more recent survey reported a detection rate of 70% when scans were performed at <14 weeks’ gestation; however, the prevalence in fetuses <11 weeks was not specified.⁹⁹ Another collaborative study conducted on fetuses with a prenatal diagnosis of LUTO <18 weeks concluded that no significant differences in the number of glomeruli generations could be identified between the early-unfavorable-prognosis group (≤18 weeks) and the group with a favorable prognosis.¹⁰⁰ Increased nuchal translucency, single umbilical artery and congenital heart defects are the most common associated abnormalities.⁹⁴

AMNION, YOLK SAC AND EARLY DEVELOPMENTAL FINDINGS 

The fetal adnexa, amnion and yolk sac can be clearly visualized as a 'see-through' snapshot from around 7 weeks' gestation, particularly using transvaginal three-dimensional ultrasound with advanced rendering techniques such as HDlive™, Realistic Vue™ and Crystal Vue™ (Figure 30). From 7–8 weeks onwards, the limb buds, developing skeletal structures and spine also become visible.

At this stage of embryonic development, ultrasound may demonstrate a range of early developmental findings involving the amnion, yolk sac and umbilical cord, including umbilical cord cysts, abnormalities of the yolk sac (such as enlargement, hydropic change or marked hypoplasia), and an abnormal relationship between the size of the embryo and the surrounding amniotic cavity.

Umbilical cord cysts (Figure 31) generally have a favorable intrauterine outcome and may resolve spontaneously during pregnancy. They are classified as true cysts and pseudocysts. True cysts have been reported in approximately 3.4% of first-trimester pregnancies, with persistence into the second trimester in around 20% of cases. The two types can be distinguished by their location: true cysts are situated close to the insertion of the umbilical cord, whereas pseudocysts are usually located more distally. Although isolated umbilical cord cysts are generally associated with a favorable outcome, an association with chromosomal abnormalities and congenital anomalies should be considered during counseling.¹⁰¹

Abnormalities of the yolk sac may also be recognized during the first trimester. A yolk sac measuring >7 mm in diameter or demonstrating hydropic enlargement (Figures 32–34) is strongly associated with aneuploidy and an increased risk of early pregnancy loss.¹⁰² Conversely, an abnormally small and hyperechogenic yolk sac (Figure 35) may also indicate abnormal early embryonic development. An abnormal relationship between the embryo and the surrounding amniotic cavity (Figure 36) represents a further early developmental finding that should prompt careful assessment of embryonic development and pregnancy viability.

MULTIPLE PREGNANCY

Conjoined twins can be seen in 1 : 50,000 to 1 : 100,000 live births^58,59 and may result from an incomplete division of the zygote between the 13th and 15th day post-fertilization.⁶⁰ The pathogenetic mechanisms are not fully understood. Conjoined twins may be secondary to a failed or delayed separation or secondary to stem cell fusion of the cotwins in monozygotic-monoamniotic twinning.^{61,62,63,64,65} With high-resolution transvaginal ultrasound, conjoined twins can be diagnosed during the first trimester, including at approximately 10 weeks' gestation.⁶⁶ (Figure 37) Cephalothoracopagus is estimated to occur in approximately 1 in 3,000,000 live births and accounts for around 1 in 58 cases of conjoined twinning. Perturbation in the maternal gene Vg1, a member of the TGF-beta family that regulates dorso-anterior development and specific actions of Hox and Pax genes during early embryogenesis, may have potential implications as one of the pathogenetic mechanisms.⁶⁷ Furthermore, Werner et al. have demonstrated that monochorionic-diamniotic quadruplet twin pregnancy may be detected as early as 11 weeks’ gestation using transvaginal three-dimensional ultrasound.⁶⁸

When monochorionic twins have an altered circulation characterized by the presence of wide artery-to-artery and vein-to-vein anastomoses that cause the exclusion of vascular perfusion of one twin (the 'acardiac twin', to be distinguished from the other, discordant cotwin, i.e. the 'pump' twin), a situation known as twin reversed arterial perfusion (TRAP) occurs. TRAP sequence is a rare complication of monochorionic twin pregnancy, occurring in approximately 2.5% of monochorionic twin pregnancies,⁶⁹⁷⁰⁷¹ and usually the acardiac twin is also affected by acrania (Figure 38).⁶⁹

Table 2 summarizes the congenital anomalies that may be detected by ultrasound between 7 and 10 weeks' gestation, together with the corresponding Carnegie stage, CRL, gestational age and associated karyotypic abnormalities.

CONCLUSION

Since the introduction of sonoembryology into prenatal imaging and following the technological development of three-dimensional ultrasound, there is a substantial overlap between findings on microscopic examination and on imaging of the embryonic and early fetal period. While the embryonic period has been classified according to 23 Carnegie stages, no such classification is used for the fetal period. The fetal period can be divided into the early fetal period (from 10⁺¹ to 10⁺⁶ weeks’ gestation) and the late first-trimester fetal period (11⁺⁰ to 13⁺⁶ weeks’ gestation). During the late first-trimester fetal period, a combined test based on the measurement of nuchal translucency and biochemical analytes (free β-hCG + PAPP-A) and enhancement of the combined test with analysis of cell-free fetal DNA, is considered the gold standard of care and is used as a screening tool for common trisomy. Prenatal ultrasound detection of congenital anomalies is possible during the embryonic and early fetal periods, and an increasing number of congenital anomalies are being detected. Although a series of congenital malformations may be correctly identified with the early first-trimester scan, caution should be exercised in targeted cases such as those with posterior fossa defects or LUTO, which require serial follow-up scans. The introduction of 3D virtual and physical models of congenital anomalies, as well as artificial intelligence, are still at an early stage, and time will be needed before these technologies can be incorporated into daily practice after demonstrating their clinical usefulness

ACKNOWLEDGMENTS

We would like to thank the following for their kind collaboration and contribution: Dr Mario Lituania, Department of Preconceptional and Prenatal Pathophysiology, Department of Obstetrics and Gynecology, E.O. Ospedali Galliera, Genoa, Italy; Dr Alessandro Cecchi and Dr Elisa Carboni, Centro Unico Regionale ASUR, Departmental 2nd Level Service of Prenatal Diagnosis, Community Hospital, Loreto, Italy.

The images and videos of 3D human embryo models were provided by the Joint MRC/Wellcome Trust (Grants MR/006237/1, MR/X008304/1 and 226202/Z/22/Z) Human Developmental Biology Resource (https://hdbratlas.org/), to whom we extend our deep appreciation and gratitude.

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