6 doctors weighed in:
I am confused about how the weeks of pregnancy are calculated. If conception occurs in week 3 of pregnancy, how could week 4 be 2 weeks post-conception? How are the weeks of pregnancy typically determined?
6 doctors weighed in

Dr. Craig Keathley
Obstetrics & Gynecology
3 doctors agree
In brief: Menstrual Weeks
Usually, the actual date of conception is unknown, therefore pregnancy is dated based on the first day of the last period.
The due date is 40 weeks from this date. Sometimes conception date is known (ie ivf pregnancy) in which case the estimated due date is 38 weeks later; for the purposes of calculation ovulation is considered as day 14 of a theoretical cycle. Early sono will confirm dating.

In brief: Menstrual Weeks
Usually, the actual date of conception is unknown, therefore pregnancy is dated based on the first day of the last period.
The due date is 40 weeks from this date. Sometimes conception date is known (ie ivf pregnancy) in which case the estimated due date is 38 weeks later; for the purposes of calculation ovulation is considered as day 14 of a theoretical cycle. Early sono will confirm dating.
Dr. Craig Keathley
Dr. Craig Keathley
Thank
1 doctor agrees
In brief: Misconception
Conception can not occur during pregnancy, not in the 3rd or any other week; it rather starts it.
The other idea is correct, conception occurs in mid-cycle, and then the 4th week of that period is 2 weeks post-conception.

In brief: Misconception
Conception can not occur during pregnancy, not in the 3rd or any other week; it rather starts it.
The other idea is correct, conception occurs in mid-cycle, and then the 4th week of that period is 2 weeks post-conception.
Dr. Carlos Satulovsky
Dr. Carlos Satulovsky
Thank
Get help from a real doctor now
Dr. Adam Levy
Board Certified, Obstetrics & Gynecology
33 years in practice
1M people helped
Continue
107,000 doctors available
Read more answers from doctors