From CANFP
Is Pregnancy Dated From Conception or Period?
By Sheila St. John
Sep 4, 2006, 20:14
Question
My dr. said the age of my baby from the length of it was 9 weeks and 5 days. Is this from conception or from my last period?
Nalini
Answer
That would be an important distinction, and you should ask your doctor which he was referring to. Actual fetal age, of course, would be dated from conception, but often they are referring to the number of weeks since your last menses. Ovulation is assumed to have occurred on day 14(which it often is not), and so two weeks is added to the actual conception date. For instance, a baby would be due 38 weeks after conception, but full term is considered 40 weeks, becasue the pregnancy is dated from the last menses. So he may have meant you conceived almost 10 weeks ago, but he (she?) probably meant you conceived almost 8 weeks ago...
Hope that makes sense!
When a woman charts her cycles, with Natural Family Planning, a real advantage is being able to accurately identify the date of conception, based on ovulation, not just estimating conception based on the last menstrual period.
Sheila St. John
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