Pregnancy dating: Why the discrepancies in pregnancy dating?

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. Why all the discrepancy in dating?


Peg Plumbo CNM

Peg Plumbo has been a certified nurse-midwife (CNM) since 1976. She has assisted at over 1,000 births and currently teaches in the... Read more

Discrepancies in pregnancy dating can be confusing. Every clinic seems to date a pregnancy a little differently.

Naegle's rule states to count back three months from the first day of the last normal menstrual period (LMP) and add seven days. Thus, a LMP of 5-12-00 becomes 2-19-00.

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. (Adding to the confusion, this gives us 40 weeks in menstrual age but 38 weeks gestational age. Keep in mind that most pregnancy "landmarks" are figured in menstrual age. For example, 40 weeks is considered a full-term pregnancy.

On a personal note, if I can delay the interventions and maternal concern about becoming "overdue" just a few more days at the end, I am happy. That is why Naegle's rule is more satisfying to me.

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