“Gestational age” is (customarily defined as) the age of a pregnancy as determined from the time of onset of the “last menstrual flow.” The assumption is that the time that it takes to make and release a mature egg at ovulation is about 14 days. If this is not so, such as when fertility medications are used to stimulate multiple egg development, then an “idealized last menstrual period” may be determined by making the day of ovulation “idealized cycle day 14” (of an ideal 28 day cycle) and counting backwards.
At 6 weeks gestational age (4 weeks after fertilization), the developing embryo has a (external) heart, an irregular body stalk, a tail, and an irregular head. On ultrasound exam, these embryos usually have a CRL (crown rump length) of about 4mm and the fetal heart can often be seen as active.
At 8 weeks gestational age (6 weeks after fertilization), the embryo is developing limb buds (arms and legs are beginning to sprout out), the heart is internalized and can be seen within a bulge, and the head is large but is beginning to take more shape. On ultrasound exam, the embryo usually has a CRL of about 17mm with an active fetal heartbeat.
From 8 weeks gestation onward, there is continued “remodeling” of the fetal head and body as well as a dramatic increase in overall size.