Due Date calculator

Estimation of Due Date

A due date-also called an estimated date of confinement (EDC) is an approximate date. When a pregnant woman is expected to give birth. It is important to remember that this is just an estimate. In fact, only about 4% of babies are born exactly on their due date, so often it is more helpful to think of a range rather than a single day.

Estimation of due date may be done in various ways, including the last menstrual period, ultrasound measurement of the embryo or fetus, conception date, and IVF transfer dates.

Last Menstrual Period (LMP)

The most common method uses the first day of a woman’s last menstrual period (LMP). That assumes that pregnancy lasts about 280 days or 40 weeks from the beginning of the last LMP. Even though it is debated whether technically pregnancy begins on fertilization, or on implantation, gestational age always counts from the LMP.

Most pregnancies will fall between 37 and 42 weeks, with 40 weeks used as the average. Using this method, the due date is calculated by counting 40 weeks from the first day of the last menstrual period.

Ultrasound

Ultrasound provides an estimated due date by measuring the size of the baby and comparing that to the average growth rates. This method is safe, quick, and painless, and is most accurate early in pregnancy. Since fetal growth in the first trimester is pretty standard, ultrasound dating can be more precise than using the LMP alone.

Conception Date

If the date of conception is known, another way to estimate due date is from that date. This calculation is similar but differs by about two weeks. The LMP method, reflecting the average time between the last menstrual period and ovulation.

(IVF) In Vitro Fertilization 

Due date estimation is generally more precise with IVF because the important dates are known. The estimate still assumes a 40-week pregnancy, but calculations can be based on dates of ovulation, egg retrieval, fertilization, or embryo transfer. Most often, the embryo transfer date (either a 3-day or 5-day embryo) is used to calculate due date.

Understanding the Due Date Range

A due date is only an estimate and not an expiration date. Babies arrive between:

37–39 weeks is considered early term
39–41 weeks are considered full term.
41–42 weeks are considered late term

Babies born before 37 weeks are considered preterm, while those born after 42 weeks are post term. Many babies born within the 37–42 week range are healthy, though full-term babies tend to have the best outcomes.

These ranges assist the doctor in deciding whether or not medical intervention is required. For instance, labor may be postponed if it has started too early so as to avoid the risk associated with prematurity. If pregnancy goes past 42 weeks, labor may be induced because the placenta may stop affording the developing baby sufficient oxygen and nutrients.