The gray scale appearance of the normal pregnancy from 4 to 16 weeks of gestation.

Since the advent of gray scale, more detail is available on sonograms and the products of conception are more readily seen. The gestational sac can be seen at about the fifth week of gestation and the fetal parts can be recognized around the seventh week of gestation. The gestational sac, fetal parts, and the placenta can be followed all the way through pregnancy without any "blind periods." In the first three months of pregnancy, gestational age can be determined by measuring the size of the gestational sac in centimeters and adding 2 to that number. From then on the head and chest measurements can be used for a more accurate determination of gestational age.