How Long Does Pregnancy Last? Pregnancy Length and Due Date

Pregnancy usually lasts about 40 weeks when counted from the first day of the last menstrual period. The estimated due date is a guide: the baby may arrive before or after it.

Pregnancy is dated from the last menstrual period, so the gestational age is about two weeks longer than the time since conception. The due date is calculated from this age and may later be adjusted based on examination results.

How are pregnancy length and the due date calculated, and why can’t they predict the exact day a baby will arrive?

• 40 weeks Average pregnancy length by gestational age
• 280 days Average pregnancy length from the first day of the last menstrual period
• from 38 to 42 weeks The period when labor may begin

How long does pregnancy last by gestational age?

Pregnancy lasts 40 weeks, or 280 days, on average when measured by gestational age: the count starts on the first day of the last menstrual period.

This count begins before conception is expected to occur. That’s why gestational age is not the same as the time the embryo has been developing since conception.

How gestational age differs from embryonic age

Embryonic age is counted from conception. But the exact date of conception is often unknown, so it is harder to calculate. Gestational age is tied to the first day of the last menstrual period—a date that is easier to use for calculations.

For everyday purposes, pregnancy length and the estimated due date are usually based on gestational age. The due date is calculated by adding 280 days to the first day of the last menstrual period. It is a guide, not a promise that labor will begin on that day.

To avoid confusing the two ways of counting, check whether the calendar or appointment lists gestational age or embryonic age.

How do you calculate the estimated due date?

Calculating from the last menstrual period

You can calculate the estimated due date (EDD) by adding 280 days to the first day of the last menstrual period. Two calendar rules are also used in obstetric calendars:

• Add 9 months, subtract 7 days: start from the first day of the last menstrual period.
• Subtract 3 months, add 7 days: apply the same steps starting from the first day of the last menstrual period.

These methods give you a guide, not the exact day of delivery. If you’re not sure when your last menstrual period began, the calculation will also be approximate: the starting date affects the EDD.

Write down the date you’re counting from and the EDD you calculate. Discuss your pregnancy length with your doctor at your antenatal clinic—especially if you don’t know when your last menstrual period began or your cycles were irregular.

How does gestational age relate to the trimesters?

Gestational age is divided into three trimesters: the first covers weeks 1–13.6, the second weeks 14–27.6, and the third includes the final weeks before delivery.

During the first trimester, a woman’s body undergoes rapid changes. In the second trimester, hormone levels stabilize and most expectant mothers feel better; the belly starts to round. The skin may stretch, sometimes causing itching.

In the third trimester, pregnancy draws closer to delivery, but pregnancy length is still expressed in gestational weeks. This scale helps track the progress of pregnancy and discuss it with your doctor.

The week number does not predict when labor will begin: the estimated due date remains a guide, not an exact promise. So even when gestational age is known, it’s impossible to determine the exact day labor will start.

When is calculating from the last menstrual period not suitable for everyone?

Calculating pregnancy length from the last menstrual period may be unreliable if the first day of the last period is unknown or cycles are irregular. In these cases, the calendar date may not accurately reflect pregnancy length.

An ultrasound helps the doctor clarify pregnancy length using measurements and compare it with menstrual dating. The doctor decides which dating to use going forward based on the specific situation and examination results.

If the calendar calculation and ultrasound give different dates, don’t choose the one that seems more convenient on your own. Discuss with your doctor which dating to use for further care.

After IVF

For a pregnancy following IVF, pregnancy length is dated more precisely using the embryo transfer date and the embryo’s age. This information is better suited to dating the pregnancy than a calculation based only on the last menstrual period.

Why doesn’t the EDD mean the baby’s exact birthday?

The EDD does not indicate the exact birthday: it is an estimated guide, and labor may begin any day from 38 to 42 weeks of pregnancy.

The EDD is calculated from the first day of the last menstrual period, and pregnancy length is clarified by ultrasound. This information helps determine how far along the pregnancy is, but it cannot predict the delivery date in advance. Even when both calculations are available, the baby may arrive before or after the EDD.

For plans tied to a specific day, don’t treat the EDD as a scheduled date. Check your pregnancy length and next steps with your antenatal clinic: your doctor will take your situation into account and advise you what to use as a guide.

Frequently asked questions

Key takeaways

• Gestational age: 40 weeks, or 280 days from the first day of the last menstrual period.
• The EDD can be calculated by adding 280 days, but it is only a guide.
• For a pregnancy following IVF, the embryo transfer date and the embryo’s age are taken into account.
• Labor may begin any day from 38 to 42 weeks.