How pregnancy weeks are counted.
Why pregnancy weeks start two weeks before conception, when each trimester begins, and how to count from a due date you were given.
By Suraj Kumar2 min read
Pregnancy weeks are counted from the first day of the last period, not from conception, which surprises many people.
From the period.
Conception is usually about two weeks after the period starts, so at 12 weeks pregnant the baby is about 10 weeks from conception. Doctors count this way because the period's date is usually known and conception's is not.
Trimesters.
The first trimester runs to 13 weeks and 6 days, the second from 14 weeks to 27 weeks and 6 days, and the third from 28 weeks.
From a due date.
If you were given a due date, choose Due date given; the weeks are counted back 280 days from it.
Further reading
Topics: pregnancy week calculator · how many weeks pregnant am i · gestational age · trimesters · weeks pregnant
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