Many expectant parents get confused about how to calculate the date on which they can expect their baby to be born. There are several reasons for this confusion. Most non-medical people think a pregnancy lasts 9 months and talk about the length of the pregnancy in months, whereas doctors and midwives measure the duration of pregnancy in weeks. Further confusion occurs over the time when a pregnancy begins.
Pregnancy actually begins at the moment of conception — when the sperm and egg meet. This must happen within a day or 2 of ovulation (release of the egg from the ovary). If pregnancy doesn't happen, a period will normally occur 2 weeks after ovulation. Thus for a woman with the normal 4-week menstrual cycle (the interval between her periods), ovulation occurs in the middle of the cycle, 2 weeks after the last period and 2 weeks before the next. However, for women who have a 6-week cycle, ovulation occurs 4 weeks after the last period, with the period still coming 2 weeks later.
In medical language, the average pregnancy lasts for 38 weeks from the date of conception. However, it is more convenient to record the date of the last period. Thus for a woman with a normal regular 28-day (4-week) menstrual cycle the pregnancy will last for 40 weeks from the last period, although of course the mother-to-be wasn't actually pregnant for the first 2 weeks. Likewise for a woman who has a regular 6-week cycle, pregnancy can be expected to last 42 weeks from the last period.
The modern technique of ultrasound enables us to measure accurately the age of the pregnancy, especially in the early stages. However, sometimes this investigation is not available and it is sensible for all women, especially those planning a pregnancy, to keep a record of the dates of periods.
Last Reviewed: 01 May 2002