Over the past decade, prenatal screening and diagnosis has moved from the second into the first trimester, with aneuploidy screening becoming both feasible and effective. With vast improvements in ultrasound technology, sonologists can now image the fetus in greater detail at all gestational ages. In the hands of experienced sonographers, anatomic surveys between 11 and 14 weeks can be carried out with good visualisation rates of many structures. It is important to be familiar with the normal development of the embryo and fetus, and to be aware of the major anatomical landmarks whose absence or presence may be deemed normal or abnormal depending on the gestational age. Some structural abnormalities will nearly always be detected, some will never be and some are potentially detectable depending on a number of factors.
Early fetal anatomical sonography
Early pregnancy ultrasound was initially introduced for the purpose of determining fetal viability, for measuring crown-rump length (CRL) to achieve accurate pregnancy dating, and determining fetal number and chorionicity, in the case of multiple pregnancy. Over the past decade, prenatal screening and diagnosis has moved from the second into the first trimester, with aneuploidy screening becoming both feasible and effective. With vast improvements in ultrasound technology, sonologists can now image the fetus in greater detail at all gestational ages. In the hands of experienced sonographers, anatomic surveys between 11 and 14 weeks can be carried out with good visualisation rates of many structures. In this review, we deal exclusively with singleton pregnancies and focus on the visualisation of the developing fetus, the feasibility of detecting normal fetal anatomy, and non-chromosomal structural anomalies in the first trimester.
Visualisation of normal fetal structures: the embryo
The fourth to the ninth week of gestation
Knowledge of the ‘normal’ development of the embryo and fetus is essential when scanning in the first trimester. The sonographer should be familiar with the major embryological landmarks during each week of gestation, as the presence or absence of certain structures may be deemed normal or abnormal depending on the gestational age of the pregnancy.
Four to five weeks
The gestational sac can first be imaged sonographically at about 4 weeks from the last menstrual period. The yolk sac is a circular structure, located between the chorion and the amnion, and first visualised at the fifth postmenstrual week. The size of the embryo ranges from 2–3 mm in size and appears as a linear structure attached to the yolk sac and close to the uterine wall. Although embryonic cardiac activity can be visualised at this time, rates of less than 100 beats per minute are not predictive of a poor outcome, and follow-up scanning is imperative.
Six to nine weeks
Descriptions of the sonographic appearance of developing anatomical structures of the embryo between 6 and 9 completed weeks are limited, with the focus on gestational sac size and crown rump length development. Much of the information relating to sonoembryology comes from longitudinal studies. The embryonic heartbeat lies within the embryo, which is an otherwise undifferentiated structure at this time. An average heart rate of 130 beats per minute can be obtained using M-mode. If the embryo is less than 4 mm, the absence of cardiac activity is non-diagnostic. Towards the end of the sixth week, the embryo is seen separately from the yolk sac. After fetal cardiac activity, the next anatomical structure to become visible is the primitive neural tube. Sonographically, this appears as a hypoechoic longitudinal structure running the length of the embryo, visible in the form of two parallel lines.
At 7 weeks gestation, the head and trunk can be visualised separately. Within the head, an intracranial cystic structure is visualised corresponding to the fourth ventricle (rhombencephalon). The cerebral hemispheres can be visualised in some embryos at this gestation. The initial sign of normal herniation of the gut can be seen as an echogenic area at the abdominal insertion of the cord.
During week 8, the choroid plexus becomes visible and grows correspondingly with the cerebral hemispheres, developing into a crescent shape traversing the roof of the fourth ventricle. The third ventricle (diencephalon) is wide. The stomach can first be visualised at this gestation as a small hypoechogenic area on the left side of the upper abdomen, and should be seen in all embryos by 11 weeks. It is possible to identify the atrial and ventricular walls of the heart moving reciprocally at the end of week 8, with the atrial component appearing larger than the ventricular component. Clear identification between the thoracic and abdominal contents is possible by the ninth week.
By 9 weeks’ gestation, the cerebral hemispheres should be visualised in all embryos, the size of the lateral ventricles increase rapidly and the third ventricle narrows. These views are best obtained in the parasagittal plane. The spine is still characterised by two echogenic parallel lines. Normal mid-gut herniation can be seen as a large hyperechogenic mass. The stomach can be seen in most embryos before 10 weeks 0 days. The oesophagus is visible as a bright streak behind the heart. The long bones, hands and feet can be first imaged at this time.
Visualisation of normal fetal structures: the embryo
The fourth to the ninth week of gestation
Knowledge of the ‘normal’ development of the embryo and fetus is essential when scanning in the first trimester. The sonographer should be familiar with the major embryological landmarks during each week of gestation, as the presence or absence of certain structures may be deemed normal or abnormal depending on the gestational age of the pregnancy.
Four to five weeks
The gestational sac can first be imaged sonographically at about 4 weeks from the last menstrual period. The yolk sac is a circular structure, located between the chorion and the amnion, and first visualised at the fifth postmenstrual week. The size of the embryo ranges from 2–3 mm in size and appears as a linear structure attached to the yolk sac and close to the uterine wall. Although embryonic cardiac activity can be visualised at this time, rates of less than 100 beats per minute are not predictive of a poor outcome, and follow-up scanning is imperative.
Six to nine weeks
Descriptions of the sonographic appearance of developing anatomical structures of the embryo between 6 and 9 completed weeks are limited, with the focus on gestational sac size and crown rump length development. Much of the information relating to sonoembryology comes from longitudinal studies. The embryonic heartbeat lies within the embryo, which is an otherwise undifferentiated structure at this time. An average heart rate of 130 beats per minute can be obtained using M-mode. If the embryo is less than 4 mm, the absence of cardiac activity is non-diagnostic. Towards the end of the sixth week, the embryo is seen separately from the yolk sac. After fetal cardiac activity, the next anatomical structure to become visible is the primitive neural tube. Sonographically, this appears as a hypoechoic longitudinal structure running the length of the embryo, visible in the form of two parallel lines.
At 7 weeks gestation, the head and trunk can be visualised separately. Within the head, an intracranial cystic structure is visualised corresponding to the fourth ventricle (rhombencephalon). The cerebral hemispheres can be visualised in some embryos at this gestation. The initial sign of normal herniation of the gut can be seen as an echogenic area at the abdominal insertion of the cord.
During week 8, the choroid plexus becomes visible and grows correspondingly with the cerebral hemispheres, developing into a crescent shape traversing the roof of the fourth ventricle. The third ventricle (diencephalon) is wide. The stomach can first be visualised at this gestation as a small hypoechogenic area on the left side of the upper abdomen, and should be seen in all embryos by 11 weeks. It is possible to identify the atrial and ventricular walls of the heart moving reciprocally at the end of week 8, with the atrial component appearing larger than the ventricular component. Clear identification between the thoracic and abdominal contents is possible by the ninth week.
By 9 weeks’ gestation, the cerebral hemispheres should be visualised in all embryos, the size of the lateral ventricles increase rapidly and the third ventricle narrows. These views are best obtained in the parasagittal plane. The spine is still characterised by two echogenic parallel lines. Normal mid-gut herniation can be seen as a large hyperechogenic mass. The stomach can be seen in most embryos before 10 weeks 0 days. The oesophagus is visible as a bright streak behind the heart. The long bones, hands and feet can be first imaged at this time.
Visualisation of fetal structures: ten to fourteen weeks – the fetus
Various protocols for first-trimester fetal anatomical ultrasound have been described. Most of these include a systematic approach, similar to that of a second-trimester anatomy scan. The aim is to obtain a transverse section of the head to demonstrate the ossified cranial bones, a midline echo, and the choroid plexuses should be seen in the ventricles; a mid-sagittal view of the face should be obtained to demonstrate the nasal bone, orbits and a normal profile; sagittal section of the spine should be determined to view the presence of intact skin over the back, and transverse and longitudinal planes of the spine from neck to sacrum; a transverse section of the thorax should be sought to demonstrate the four-chamber view of the heart with a normal axis; transverse and sagittal sections of the trunk and extremities should be obtained to demonstrate the stomach in the left upper quadrant, the bladder, the abdominal insertion of the umbilical cord, and all the long bones, hands and feet. To obtain a realistic insight into the clinical efficiency of the 11 to 13+6 week scan for the diagnosis of congenital abnormalities in an unselected or low-risk population, a literature search was conducted to identify studies of greater than 1000 pregnancies between 2002 and 2011. These results are presented in Table 1 . Smaller studies and case reports were included to obtain an overview of rarer anomalies and the demonstration of new diagnostic markers.
| Fetal abnormality | Syngelaki et al., 2011 | Ebrashy et al., 2010 | Chen et al., 2008 | Dane et al., 2007 | Weineret al., 2007 | Saltvedt et al., 2006 | Taipale et al., 2004 | Carvahlo et al., 2002 | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| N | % | N | % | N | % | N | % | N | % | N | % | N | % | N | % | |
| Total overall | 488/44859 | 1.1 | 31/2876 | 1.1 | 63/3949 | 1.6 | 24/1290 | 1.9 | 22/1723 | 1.3% | 371/18053 | 2.1% | 33/4855 | 0.7% | 130/2853 | 4.6 |
| Total in first trimester | 213/448 | 43.6 | 20/30 | 66.7 | 24/56 | 42.8 | 16/20 | 80.0 | 9/22 | 40.9 | 66/176 | 38.0 | 5/30 | 16.6% | 29/130 | 22.3 |
| Neural tube | ||||||||||||||||
| Anencephaly | 29/29 | 100.0 | 3/3 | 100.0 | 3/3 | 100.0 | 4/4 | 100 | 1/1 | 100.0 | 8/10 | 80.0 | 0/1 | 0.0 | 4/4 | 100.0 |
| Encephalocoele | 0/0 | 0.0 | 1/1 | 100 | ||||||||||||
| Open Spina bifida | 3/21 | 14.3 | 1/1 | 100.0 | 2/3 | 66 | 1/11 | 9.0 | 0/2 | 0.0 | 1/4 | 25.0 | ||||
| Sacrococcygeal teratoma | 0/1 | 0.0 | ||||||||||||||
| Brain | ||||||||||||||||
| Microcephaly | 0/1 | 0.0 | ||||||||||||||
| Craniosynostosis | 0/1 | 0.0 | ||||||||||||||
| Agenesis of the corpus callosum | 1/10 | 10.0 | ||||||||||||||
| Ventriculomegaly | 1/11 | 9.0 | 0/2 | 0.0 | 0/1 | 0.0 | 1/3 | 33 | 2/4 | 50.0 | 0/10 | 0.0 | 0/3 | 0.0 | 1/10 | 10.0 |
| Holoprosencephaly | 2/3 | 66.6 | 1/1 | 100.0 | 1/1 | 100.0 | 2/2 | 100 | 1/1 | 100.0 | 3/4 | 75.0 | 2/2 | 100.0 | ||
| Cerebellar hypoplasia | 0/3 | 0.0 | ||||||||||||||
| Vermian agenesis | 0/4 | 0.0 | 1/1 | 100 | ||||||||||||
| Face | ||||||||||||||||
| Facial cleft | 1/20 | 5.0 | 1/2 | 50.0 | 2/2 | 100.0 | 0/34 | 0.0 | 1/3 | 33.3 | ||||||
| Lungs | ||||||||||||||||
| Diaphragmatic hernia | 4/8 | 50.0 | 0/1 | 0.0 | ||||||||||||
| Congenital cystic adenomatoid malformation | 0/4 | 0.0 | 0/1 | 0.0 | ||||||||||||
| Extralobar sequestration | 0/2 | 0.0 | ||||||||||||||
| Abdominal wall | ||||||||||||||||
| Gastroschisis | 19/19 | 100.0 | 2/2 | 100.0 | 2/2 | 100.0 | 4/5 | 80.0 | 0/1 | 0.0 | ||||||
| Omphalocele | 60/60 | 1/1 | 100.0 | 1/1 | 100.0 | 3/5 | 60.0 | 1/2 | 50.0 | |||||||
| Bladder extrophy | 0/1 | 0.0 | ||||||||||||||
| Gastrointestinal tract | ||||||||||||||||
| Osophageal atresia | 0/2 | 0.0 | 0/1 | 0.0 | ||||||||||||
| Duodenal atresia | 0/2 | 0.0 | 0/1 | 0.0 | ||||||||||||
| Bowel obstruction | 0/1 | 0.0 | 0/6 | 0.0 | ||||||||||||
| Anal atresia | ||||||||||||||||
| Renal | ||||||||||||||||
| Megacystis | 29/29 | 100.0 | 2/2 | 100.0 | 1/1 | 100.0 | 1/2 | 50.0 | 5/5 | 100.0 | 1/1 | 100.0 | 3/3 | 100.0 | ||
| Renal agenesis | 1/7 | 14.3 | 2/2 | 100.0 | 0/2 | 0.0 | 0/2 | 0.0 | ||||||||
| Hydronephrosis | 1/11 | 9.0 | 1/5 | 20.0 | 1/6 | 16.6 | ||||||||||
| Multicystic dysplatic | 1/17 | 5.8 | 0/1 | 0.0 | 1/1 | 100 | 0/2 | 0.0 | 1/7 | 14.3 | 0/1 | 0.0 | ||||
| Infantile polycystic kidneys | 2/6 | 33.3 | ||||||||||||||
| Duplex kidneys | 0/12 | 0.0 | ||||||||||||||
| Skeleton | ||||||||||||||||
| Lethal skeletal dysplasia | 3/6 | 50.0 | 1/1 | 100.0 | 1/1 | 100.0 | 0/1 | 0 | 0/2 | 0.0 | 2/4 | 50.0 | 1/1 | 100.0 | ||
| Achondroplasia | 0/1 | 0.0 | ||||||||||||||
| Arthrogryphosis | 0/2 | 0.0 | ||||||||||||||
| Talipes | 4/38 | 10.5 | 1/1 | 100.0 | 1/12 | 8.3 | 1/27 | 3.7 | ||||||||
| Short long bones | 2/4 | 50.0 | ||||||||||||||
| Absent hand/foot (limb reduction) | 7/9 | 77.8 | 1/1 | 100 | 2/12 | 16.6 | 0/2 | 0.0 | ||||||||
| Polydactyly | 12/20 | 60.0 | 0/3 | 0.0 | 0/2 | 0.0 | ||||||||||
| Other | ||||||||||||||||
| Body stalk anomaly | 5/5 | 100.0 | ||||||||||||||
| Hydrops | 1/1 | 100.0 | 0/2 | 0.0 | 1/3 | 33.3 | 1/1 | 100.0 | ||||||||
| Cloacal defect | 0/1 | 0.0 | ||||||||||||||
| Other/multiple | 18/83 | 21.6 | 2/6 | 33.3 | 6/23 | 26.1 | 1/2 | 50.0 | 3/30 | 10.0 | 0/3 | 0.0 | 6/28 | 21.4 | ||
| Cystic hygroma | 7/9 | 77.8 | 5/5 | 100 | 3/3 | 100.0 | ||||||||||
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