(English) The Normal First Trimester Fetus

Several ultrasound studies using mainly the transvaginal approach at 4 to 12 weeks of gestation describe the developent of the normal fetus as fallows:

1. The 4th Week:

   The gestational sac is visible from about 4 weeks and 3 days as a hypoechoic, ring-like2 to 3 mm structure surrounded by a hyperechoic rim. The sac represents the chorionic cavity and is typically located in the upper part of the decidualized endometrium in an eccentric position.

2. The 5th Week:

   The yolk sac becomes visible at 5 weeks. At the end of the 5th week, the fetal pole measures 2 to 3 mm ; we can also identify the yolk sac and the heart activity. The fetal heart rate is about 110 beats per minute.

3. The 6th Week:

   The embryo measures 4 to 8 mm (CRL=crown-rump length). We identify the fetal pole, the yolk sac and the heart activity. The heart activity should always be seen in embryos of 5 mm ( 6 weeks and 4 days). The gestational sac contains two fluid-filled cavities: the celomic cavity (chorionic) with the yolk sac ; the amniotic cavity who is smaller and contains the fetal pole. The two cavities are separated by a thin membrane surrounding the embryo (is the amniotic membrane ).
   At the end of the 6th week, the cavity of the rhombencephalon becomes visible as a small hypoechogenic area at the cephalic part of the fetal pole.

4. The 7th Week:

   CRL is 9 to 14mm. The heart rate is about 130 beats per minute. The amniotic cavity starts expanding rapidly. The head can be distinguished from the body and hypoechogenic area (brain vesicles) appear in the developing brain at the cephalic end of the embryo. At this stage, the rhombencephalon is larger than the developing hemispheres. The small cavities of the hemispheres originating from the dividing Y-shaped telencephalon, the diencephalon ( who’ll form the thalami, hypothalamus and the 3rd ventricle ) and the mesencephalon (that will form the nuclei and the aqueduct of Sylvius ) may be identifiable.
   The umbilical cord is short and appears wider and hyperechogenic at the point of the insertion in the abdomen ; this is the first sign of bowel herniation into the cord. The lower limb buds can vaguely be depicted.

5. The 8th Week:

   CRL is 15 to 22 mm. The brain vesicles are more proeminent and the choroid plexus can be visualized as small hyperechogenic areas. The rhombencephalon is still the largest cavity lying on top of the brain. There is a connection between the lateral ventricles, the diencephalon and the mesencephalon.
   The fetal heart rate is about 160 beats per minute and the heart occupies more than 50% of the thoracic cavity. Atria and ventricles are sometimes visible at the end of the 8th week.
   The abdominal cavity is fully occupied by the liver anteriorly and the stomach dorsally while the intestine is herniated into the umbilical cord. Fluid in the stomach can occasionally be seen toward the end of the 8th week probably as a product of the gastric epithelium. (The swallowing movements starts just at 11 weeks ).
   The outlines of the skull, spine and ribs can vaguely be seen. The limbs appear as short echogenic outgrowth. The first body movements become visible during the 8th week.

6. The 9th week:

   CRL is 23 to 31 mm. The amniotic cavity is larger than the celomic cavity and occupies most of the sac volume. The fetal body becomes ellipsoid with the head being disproportionally big and the soles of the feet touch in the the midline. The choroid plexus are obvious and occupy almost fully the lateral ventricles. The cortex is thin and hypoechoic. The cerebral hemispheres become visible and are clearly separated. The connection between the mesencephalon and the third ventricle becomes narrower.
   The fetal heart rate reaches a peak of about 175 beats per minute.
   Bowel herniation is visible in all fetuses.
   The first ossification centres can be seen at the mandible and clavicle.
   Limb movements can be identified.