When I went for my first prenatal visit, it was exactly 10 weeks from my last period, but 8 weeks from the actual conception date. An ultrasound two days later showed I was in my ninth week. She has assisted at over 1,000 births and is currently teaching in the nurse-midwifery program at the University of Minnesota. Discrepancies in pregnancy dating can be confusing.
Every clinic seems to date a pregnancy a little differently.
Pregnancy, just as the menstrual cycle, is based on the lunar month, which has 28 days. Because of this, we eliminate two to three days per month from our counting. In addition, the 40 weeks is calculated from the first day of the last menstrual period, so we are actually counting 14 days when the woman isn't even pregnant. That is why Naegle's rule is more satisfying to me. Find out here.