At your first prenatal appointment, your doctor will determine your due date by asking the date of your last menstrual period (conception takes place about two weeks after your last period). To calculate the your due date, the doctor will have to subtract three months from the first day of your last menstrual period and add seven days.
For example, if the first day of your Last Menstrual Period (LMP) was May 5, count back 3 months then add 7 days – the Estimated Due Date (EDD) is February 12 of the following year.
Month Day Year LMP May 5 2008 -3 +7 +1 EDD Feb 12 2009
Pregnancy lasts an average of 280 days or 40 weeks. When your doctor says you are twelve weeks pregnant, then it means that the fetus is ten weeks old.
If you cannot remember the date of your last menstrual period, or if your last period was scant or unusual, the doctor will have to use other methods of calculating your due date. It maybe:
- Throughout pregnancy, but especially early on, the size of the uterus can provide useful information.
- Fetal heart tones can usually be heard at 12 weeks by using a Doppler Device, a form of ultrasound that converts sound waves into signals you can hear, or at 18-20 weeks by using a special stethoscope.
- Yu can usually feel first fetal movements, or quickening, by 16-20 weeks.
- In the first half of pregnancy, ultrasound can be used to estimate the age of a fetus within 7-10 days. Later, this method in not as accurate.
Note: Remember, normal pregnancies vary in length. Think of your due date as approximate, and expect the baby from two weeks before or two weeks after that date. Two thirds of all babies are born within ten days of their due dates, but only 4% are actually born on their due date.