Why pregnancy is counted from a period

The usual dating convention begins on the first day of the last menstrual period, rather than the day of conception. It assumes a roughly 28-day cycle and includes approximately two weeks before fertilization. Individual cycles and dates vary.

  • The estimated due date is conventionally 280 days, or 40 weeks, from the period's first day.
  • If the period date is unknown or cycles are irregular, ask your care team how your pregnancy is being dated.

Weeks and days: an example

If the first day of a period was January 4, 2027, then April 1, 2027 is 12 weeks and 3 days later. The conventional due-date estimate is October 11, 2027. A clinician may establish a different date using the information available for that pregnancy.

  • 12 weeks and 3 days means 12 completed weeks plus 3 days.
  • Niva labels this as week 12; some resources use different week-number labels, so compare the weeks-and-days value.

When each trimester starts

ACOG describes the first trimester through 13 weeks and 6 days, the second from 14 weeks through 27 weeks and 6 days, and the third from 28 weeks onward.

  • First trimester: through 13 weeks and 6 days.
  • Second trimester: 14 weeks through 27 weeks and 6 days.
  • Third trimester: starts at 28 weeks.

What if my dates do not match?

A last-period calculation is an estimate. Your care team may use ultrasound and your history to establish a due date. For IVF, ACOG recommends using the embryo's age and transfer date to assign the due date.

  • Use the due date your clinician or fertility team has documented for your care.
  • Ask which date to use if your calculator, ultrasound or treatment records differ.