Even though conception usually happens about two weeks after the first day of your last period, pregnancies are dated from the first day of your last period. This can get confusing when, say, your doctor tells you that the ultrasound shows your baby is nine weeks and two days old, which makes you 11 weeks pregnant. Why are pregnancy calendars so weird?
Basically, the first-day-of-last-period (FDLMP) calendar is a relic of an earlier time, before ultrasounds in early pregnancy were able to date a pregnancy pretty closely to the exact day of conception. Because everyone’s cycle is different, and cycles are not completely predictable, the first day of a woman’s last period would be the one day that everyone could be absolutely certain that she was not pregnant. By this method, a due date is established by adding a year, then subtracting three months, then adding seven days to the first day of your last period. If you know that your cycle is usually longer or shorter, your health care provider may also factor in your adjusted cycle length.
If you happen to know the date of conception, another way to calculate your due date is to count ahead 38 weeks on a calendar. But note that even this is not an exact science, the length of a baby’s gestation will depend on a lot of factors: first babies, European-American and male babies have an average gestation that’s as much as a week longer, babies whose mothers have African or South American genetic heritage, are female or have older siblings will tend to arrive closer to the actual estimated due date. Health issues can change the length of gestation also. And ultrasounds also become less accurate as time goes on– if you discover that you’re pregnant in the second trimester or later, an ultrasound estimate of the baby’s age could be off by weeks.
If you don’t happen to remember the first day of your last period (and let’s face it, not all of us are meticulous calendar keepers), then your health care provider will usually schedule and ultrasound.