Timing and duration of your pregnancy

Timing in Pregnancy

Antepartal period

Covers the time of pregnancy from the first day of the last normal menstrual period (LMP) to the start of true labor, which marks the beginning of the intrapartal period.


Prenatal period

Covers the time of pregnancy from the first day of the last normal menstrual period (LMP) to the birth of the baby, which marks the beginning of the postnatal period.

Divided into three trimesters:

  • Each approx. 13 weeks or 3 calendar months

First trimester

– 1 to 12th week (Last 12 weeks)

Second trimester

– 13 to 27th week (Last 15 weeks)

Third trimester

– 28 to 40th week (Last 13 weeks)

Related How to estimate your date of delivery (EDD)

Duration of pregnancy

From the first day of the LMP
(considered day one of pregnancy)

  • 280 days
  • 40 weeks
  • 9 calendar months
  • 10 lunar months

A normal/regular (same length every month specifically for you) menstrual cycle starts on the first day of your last period and ends on the first day of your next period (menstrual cycle isn’t the same for every woman). Your menstrual cycle might be somewhat irregular, long or short, painful or painless and your period might be light or heavy and still be considered normal.

The average cycle is 28 days long, anything between 21 and 45 days (24-day difference) is considered normal as well. How long your period lasts vary. The time from the first sign of blood to the last is usually in the 3 to 7-day range. Anything from 2 to 7 days period is normal. For the first few years after menstruation begins, long cycles are common. However, menstrual cycles tend to shorten and become more regular as you grow older.

Remember that use of certain types of contraception can alter your menstrual cycle.

In menopause, your cycle might become irregular again and then stop, at this time you won’t be able to have more children.

From the day of fertilization

  • 266 days
  • 38 weeks

(Adding 14 days gives a total of 280 days from the LMP) If we believe your due date is off target, we will measure your baby during an ultrasound, and if your baby is bigger or smaller than expected, we’ll provide you with a new due date. Your due date is just an estimate. Only 1 in 20 women delivers on her due date. But it’s important so we can follow your baby fetal development. You have to know that you can go into labor any day during the two weeks before or after your due date.

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