The majority of women don’t know the exact date of conception, but usually know the beginning of their last period. So, this is the point from which a pregnancy is dated. A due date helps your doctor verify when to do certain tests or procedures. The due date also helps determine your baby’s growth and may point out whether you’re overdue.
Pregnancy lasts about 40 weeks (or 280 days), from the beginning of your last period. You can find out what your due date is by counting 280 days from the first day of bleeding of your last period. Or count back 3 months from the date of your last period and add 7 days. This also gives you an estimated date of delivery. For example, if you last period began on February 20, your due date is November 27. Estimating a pregnancy using this method gives the gestational age. This is how most doctors and nurses keep track of time during pregnancy.
Many people count the time during pregnancy using weeks. It’s really the easiest way. You may hear people talk about what stage of pregnancy you’re in using trimesters. Trimesters divide pregnancy into three separate time frames, each about 13 weeks long. This helps group together developmental stages. As an example, your baby’s body structure is mostly formed and their organ systems develop during the first trimester.
You may also hear about lunar months, referring to a complete cycle of the moon (which is 28 days). Because pregnancy is 280 days from the beginning of your period to your due date, pregnancy lasts 10 lunar months.