What Gestational Age Tells You About Your Delivery Date

Medically reviewed by Peter Weiss, MD

Gestational age is the number of days and weeks that measure the length of pregnancy. It helps healthcare providers determine your due date. It is calculated by counting from the first day of the last menstrual period. This is different from fetal age, which is the number of days and weeks from the day of conception.

Learn about gestational age, how to calculate it, tracking pregnancy, and more.

Gestational Age: Not Technically Baby’s Age

When people ask, "How far along are you?," most people respond with the gestational age. The gestational age is the number of days and weeks of pregnancy beginning on the first day of the last menstrual period. Gestational age measures the age of gestation (pregnancy), not the age of the fetus.

Conception occurs during (and shortly after) ovulation, a time frame known as the fertility window. For people with a 28-day cycle, ovulation occurs approximately two weeks after the first day of the last menstrual period.

Experts use a 28-day cycle to calculate gestational age based on the average menstrual cycle length. However, a "normal" cycle lasts anywhere from 23 to 35 days, and not all menstruating people fall within that range. People generally on a 28-day cycle can experience occasional changes and may ovulate early or late.

Since gestational age is based on the date of your last period—and not the date of conception—you are considered four weeks pregnant at the time of your first missed period.

How to Calculate Gestational Age

When calculating the gestational age, the first day of your last menstrual period is considered day one. After that, it is generally counted in weeks plus the number of days past the previous week. For example, the gestational age could be 16 weeks and three days, meaning three days past 16 weeks.

There are online gestational age calculator tools that work by entering the current date and the date of the start of your last menstrual period. These calculators may also provide an estimated due date to predict the baby's birth date.

Use this date as an estimate only, especially if you have not been tracking your menstrual period. The first day of the last menstrual period is accurately remembered and used only about half the time.

Tracking Estimated Delivery Date With Gestational Age

The estimated delivery date of the baby is at 40 weeks of pregnancy. However, only about 16% of menstruating people have a 28-day menstrual cycle length, and conception does not happen on an exact day during the menstrual cycle.

This means that even when you accurately remember or track the first day of the last menstrual period to calculate gestational age, the due date is not a precise calculation. Ultrasounds during the first trimester of pregnancy are the preferred method of measuring the age of the baby. Regardless of how the due date is calculated, the baby may be born before or after that date.

How Gestational Age Is Used During Pregnancy and After Birth

Healthcare providers use gestational age to measure pregnancy progress and monitor for potential complications. It is referenced regularly throughout pregnancy.

If you ever suspect you are pregnant, seek the support of a healthcare provider, such as a specialist called an obstetrician-gynecologist (ob-gyn). They can help prevent and treat any possible complications and improve pregnancy, birth, and postpartum outcomes for you and your baby.

Babies born before 37 weeks of gestation are considered premature, which happens in up to 16% of pregnancies. Babies born prematurely may hit milestones later than full-term babies in the first year of life and sometimes beyond.

For this reason, premature babies may be assessed for milestones after birth, such as sitting up, walking, and talking, based on their gestational age instead of their actual age. For example, a baby born two months early may be expected to reach 6-month-old milestones by 8 months old instead of 6.

Summary

Gestational age is the number healthcare providers use for the length of pregnancy and estimate the baby's due date. However, it is not technically the age of the baby. This is because the gestational age is calculated starting with the first day of the last menstrual period. Conception generally happens about two weeks after that, but it may occur earlier or later than this average.

While the estimated delivery date is 40 weeks gestation, that estimate is not always accurate. The menstrual cycle length can vary, conception can happen on different days throughout the cycle, and babies may be born early or late for various reasons. Providers may use gestational age to assess the developmental milestones of babies born before 37 weeks gestation.