Calculating Your Due Date 
Brittany Peats

Once you know that you are pregnant, most people want to find out right away when their due date is. The babies are born on their precise due date only about 5 percent of the time, but most of the time the baby is born within two weeks of the due date. Use a due date as a rough estimate; some doctors prefer to say that you will be born “towards the end of February” or “towards the beginning of October” so the woman has looser expectations and does not worry needlessly approaching a single day. You can figure out your expected due date yourself if you know the first day of your last menstrual period, or the day of conception. Your due date is typically figured as being 266 days from conception, or about 280 from your LMP. This is 40 weeks or 9 months (give or take a few weeks). This calculation is based on the findings by Dr. Naegle, around 1850, who determined that the average length of human gestation was approximately 266 days and he assumed that the average woman had cycles that lasted 28 days and that she ovulated on day 14 of her cycle. He used his data to come up with a mathematical calculation for due dates:

(Last Menstrual Period + 7 days) - 3 months = Due Date 
EX: (January 1, 2007 + 7 days) - 3 months = October 8, 2007

There were certain factors the Dr. Naegle omitted to take into consideration: not every woman’s cycle is 28 days, not every woman ovulates on Day 14, ethnicity, how many successful previous pregnancies the woman has had, prenatal care, better nutrition, and screening factors. One recent study indicates that we need to add 15 days to the due date that Naegele constructed for Caucasian, first time moms, and 10 days for Caucasian moms having subsequent children. African American and Asian women tend to have shorter gestations. Also, with the advent of true prenatal care, midwives and physicians are helping women educate themselves about risk factors, nutrition and prenatal screening which extend the lengths of gestation for many women.

There are also many online due date calculators which use the date of your last menstrual period or the date of conception to figure out your due date. Click on the link below to calculate your due date online.

http://healthlink.mcw.edu/article/923526833.html

There are other ways to determine the due date if you don’t remember your last menstrual period or if you have an irregular cycle. An ultrasound, if performed in the first half of the pregnancy, can be accurate. Other ways to estimate a due date include the first time the mother feels the baby move. This is called quickening and for women who are expecting their first baby, you can expect to feel your baby between 18 and 24 weeks gestation. If this is not your first baby you can expect to feel your baby a bit sooner than you felt your first. Fetal heart tones heard through doppler and stethoscope can help determine a due date, as can fundal height -- measurement of the uterus done throughout pregnancy.