The typical pregnancy lasts between 38 and 42 weeks with the average pregnancy length lasting 40 weeks, or 280 days, according to the American Congress of Obstetricians and Gynecologists. Your baby's due date is referred to as the estimated date of delivery or sometimes, the gestational age of your fetus. Either way, knowing your baby's due date helps to you to maintain your health, provide the best care for your growing baby and plan for the big day. Your baby's due date is an estimation, but there are several calculation methods to improve its accuracy.
Step 1
Determine the date of the first day of your last menstrual period. From that date, count forward 40 weeks to calculate your baby's estimated due date. Conception usually occurs about two weeks after the first day of your last menstrual period. Only about 5 percent of baby's are born on their due date, so this calculated due date is an estimate, reports the American Pregnancy Association.
Step 2
Schedule an appointment with your obstetrician. At your first prenatal visit, your physician will give you your expected delivery date, which he will calculate based upon your last menstrual period.
Step 3
Use an online pregnancy due date calculator. An online due date calculator such as the one found one at MayoClinic.com, will do the mathematical calculation for you. Some of the more advanced pregnancy due-date calculators will take into account your menstrual cycle length when performing the calculation.
Step 4
Obtain an ultrasound. Ultrasounds are the most reliable for calculating your baby's due date when conducted early in the pregnancy. An ultrasound between the 8th and 18th week of pregnancy is the best time to estimate gestational age, according to American Pregnancy Association.
Step 5
Ascertain the size of your uterus. Your baby's gestational age can be assessed by the size of your uterus, according to The American Congress of Obstetricians and Gynecologists. In early pregnancy, your obstetrician measures the size of your uterus by how big it feels to her, which will give her a rough estimate of the baby's gestation. During mid to late pregnancy, your doctor measures your uterus in centimeters using a measuring tape. The uterus size typically equals the number of week's gestation.
Tips and Warnings
- If you know the actual date of conception, you will have the most accurate information to calculate your expected due date.
- In some cases, the gestational age calculated from the size and height of the uterus does not match the calculation using the last menstrual period. An unusually small or large baby may make it difficult to pinpoint the exact due date. Conditions such as Preeclampsia, which affects roughly 5 percent to 8 percent of all pregnancies, according to the Preeclampsia Foundation, may impact the expected delivery date.
