Estimating Your Due Date

One of the most frequent questions you may hear from friends is "How far along are you?" or "When you are due?"  Your health care provider will estimate the gestational age of your baby at one of your first prenatal appointments. While this is just an estimate (only about 5% of babies are born on their estimated due date), it is important to know the approximate gestational age when monitoring your pregnancy.

Gestational age, or the age of the baby, is calculated from the first day of the mother's last menstrual period. The following information about gestational age has been adapted from the University of Michigan and American Pregnancy websites.

What is gestational age?

Gestational age is the age of an unborn baby.  It is measured in weeks and days, not in months.  Since the precise date of conception is seldom known, the age is based on the date of the mother's last menstrual period.  Forty weeks after the first day of the last menstrual period is the estimated end of a normal term pregnancy, or due date. 

Why is it important to know the gestational age?

The progress of a pregnancy can be judged as normal or abnormal only when the gestational age is accurately known.  Gestational age is very important when complications occur in the pregnancy and early delivery may be necessary.  It is also important to know gestational age so that an overdue baby can be carefully monitored. 

How is gestational age calculated?
 
The gestational age of a baby may be calculated from the date of your last period.  If you are sure of this date and have regular periods, your due date is estimated to be 40 weeks from the first day of your last period.  An early exam of the uterus and an early positive pregnancy test also help determine gestational age.

The most accurate way to determine gestational age is a reliable date for the last menstrual period confirmed by an ultrasound exam.  The baby can be measured with ultrasound as early as 5 or 6 weeks after the mother's last menstrual period.  This method is most accurate in early pregnancy.  It becomes less accurate late in pregnancy.  The best time to date a pregnancy with ultrasound is between the 8th and 18th weeks of pregnancy.

Difficulties in Determining Gestational Age

It can be difficult to determine accurately a baby's gestational age and your due date if: 

Ultrasound

The baby can be measured as early as 5 or 6 weeks after the mother's last menstrual period. Measuring the baby using ultrasound is most accurate in early pregnancy. It becomes less accurate later in pregnancy. The best time to estimate gestational age using ultrasound is between the 8th and 18th weeks of pregnancy. The most accurate way to determine gestational age is using the first day of the woman's last menstrual period and confirming this gestational age with the measurement from an ultrasound exam.

Calculating Due Date

Estimated Due Date: Based on the last menstrual period, the estimated due date is 40 weeks from the first day of the period. This is just an estimate since only about 5% of babies are born on their estimated due date.

To help you calculate your due date, use this pregnancy calendar.

Calculating Conception Date: In a Typical Pregnancy: For a woman with a regular period, conception typically occurs about 2 weeks after the first day of the last period. Most women do not know the exact date of conception, and their conception date is merely an estimate based on the first day of their last period.