DUE DATE Calculating the due date 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.
If pregnancy doesn't happen, a period will normally occur 2 weeks after ovulation. 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. Likewise for a woman who has a regular 6-week cycle, pregnancy can be expected to last 42 weeks from the last period.