In the first trimester, the gestational sac size is determined by measuring the average diameter of the bag. This is equal with the sum of cranio-caudal size, transverse dimension, anterior-posterior dimension, all divided by 3.
With endovaginal transducer, the gestational sac can be detected from 2-3 mm (approximately 4 weeks of amenorrhea). With the help of transabdominal transducer the gestational sac can be detected at 5 mm (ie 5 weeks of amenorrhea).
Normal appearance of gestational sac is a fluid collection (transonic area represented by chorionic fluid) surrounded by a hyperechogenic ring (determined by chorionic villi under development).
Detection of gestational sac, with transvaginal echography corresponds to a level of HCG of 500 -1500 UI/L.
Unfortunately, other conditions, often pathological, may determine occurrence of intrauterine fluid collection, so the ultrasound differential diagnosis should be done with:
– endometrial cyst;
– cervical stenosis;
– sac of ectopic pregnancy.
Embryonic structures (embryo echo) will not be viewed by transabdominal ultrasound only when gestational sac size reaches 15 mm (ie 6 weeks of amenorrhea). First elements that are viewed are: yolk sac and amniotic sac. Embryo echo can be detected when its length is 24 mm (5-6 weeks of amenorrhea), so once appeared, the embryo length will be measured.
When the embryo reaches 5 mm long, it can be seen very well as a separate structure of yolk sac wall, with pulsatile echo view (visible heart beats proving that pregnancy progresses). These dimensions of embryo correspond to 6 weeks of amenorrhea and a gestational sac size of 15-18 mm.
When the size of the embryo reaches 12 mm in length, using ultrasound, the cephalic extremity can be differentiated from the rest of the embryo structures, the embryo skull represents half of its total volume.
When the embryo reaches a length of 30 -35 mm (11 weeks of amenorrhea) it turns to fetus and it can be visualized the limb buds, umbilical cord, centers of ossification of the mandible, clavicle and maxillary.
2. Assessing gestational age
Estimation of gestational age puts two problems: the first one represents the accuracy of measurements, and the second one are the measurements that give a more accurate assessment of gestational age.
As measurements are performed early in pregnancy, the gestational age assessment is closer to reality.
Up to 5 to 6.5 weeks of amenorrhea, assessing accurately the pregnancy gestational age is made by sac measuring.
At 2 to 4 weeks corresponds to 3 mm.
At 5 weeks corresponds to 5 mm.
After 6.5 weeks of amenorrhea pregnancy gestational age assessment is made by measuring the length of the embryo from cephalic extremity to the distal extremity.
At 6.5 weeks of amenorrhea, the embryo has a length of 5 mm and heart pulsations can be viewed.