Each trimester lasts approximately three months:
The unborn baby spends around 38 weeks in the uterus (womb), but the average length of gestation is calculated as 40 weeks. This is because pregnancy is counted from the first day of the
woman's last period, not the date of conception which generally occurs two weeks later. Since some women are unsure of the date of their last menstruation (perhaps due to period irregularities), a
pregnancy is considered full-term if the birth falls between 37 to 42 weeks of estimated last menstrual period. A baby born prior to week 37 is considered premature, while a baby that still hasn't
been born by week 42 is said to be overdue. In many cases, labor will be induced in the case of an overdue baby.
The medical term for the due date is Estimated Date of Confinement (EDC):
However, only about 4% of women actually give birth on their EDC. There are many online pregnancy calculators that can tell you when your baby is due, if you type in the date of the first day of
your last period.
A simple method to calculate the due date is to add seven days to the date of the first day of your last period, then add nine months. For example, if the first day of your last period was
February 1, add seven days (February 8) then add nine months, for a due date of November 8.
Irregular menstrual cycles sometimes mean that a woman cannot be sure of when she conceived. Some clues to the length of gestation include:
- Ultrasound examination (especially when performed between 6 and 12 weeks).
- Size of uterus on vaginal or abdominal examination
- The time fetal movements are first felt (a very approximate guide only).