The age of an embryo or fetus is time elapsed since conception. In the past, the date of conception was almost never known, and the traditional method of dating a pregnancy is to start counting from the first day of the last period, which works when conception happens on the 14th day, right after the most frequent time of ovulation. "Menstrual" dating" is reported as gestational age (GA), which starts at 2 weeks. Average delivery at 40 weeks GA. Dividing the weeks by 4 gives you the month of the pregnancy.
Ultrasound is the best way to determine age, when the exact date of conception is not known. There is very little variation in the way healthy embryoes and fetuses grow during the first half of pregnancy - all around the world - it's a basic human norm. Tthere are excellent correlations between fetal size and conceptual age, permitting conceptual age estimation with an accuracy of just a few days. Knowing the exact fetal age is important, because it enables prevention of postmaturity (pregnancy beyond 42 weeks GA).
