Gestational age: what it means and how your weeks of pregnancy are counted

Gestational age is how far along your pregnancy is, in weeks and days. Cleveland Clinic says it goes by the first day of your last menstrual period, so the count includes roughly two weeks before you conceived. A scan report might say 22 3/7, which Cleveland Clinic explains means 22 weeks and 3 days.

Why the count starts before you were pregnant

It feels odd to be four weeks pregnant when you only found out yesterday. Cleveland Clinic explains it: ovulation happens about two weeks into your cycle, so by the time a pregnancy test turns positive, two weeks after ovulation, you are already counted as four weeks along.

Cleveland Clinic also points out that gestational age measures the pregnancy itself. Doctors rarely use the baby's age from conception because that day is so hard to pin down, so everything is anchored to your LMP, a date you can usually check on your phone or calendar.

How doctors measure it

Your period date gives the first estimate, and ultrasound checks it. KKH's obstetrics team says the date can be corrected after the first major scan to date your baby, which can happen from week 7, and KKH does this dating scan in the first trimester. Once it is done, your gestational age and your EDD move together.

Cleveland Clinic says your provider confirms your due date at your appointment or offers a new one based on the ultrasound, and keeps an eye on it at later visits.

Early term, full term and late term

The American College of Obstetricians and Gynecologists (ACOG) splits the weeks around birth into labels: early term is 37 weeks 0 days to 38 weeks 6 days, full term is 39 weeks 0 days to 40 weeks 6 days, late term is 41 weeks 0 days to 41 weeks 6 days, and postterm is 42 weeks and beyond.

In Singapore you will mostly hear one cut-off, because HealthHub says that from 37 weeks your baby is considered fully matured, or full term. Cleveland Clinic defines preterm labour as labour that begins before the 37th week, which is where premature birth begins.

Where your gestational age changes your care

Your week count decides which tests you get and when. KKH, for example, screens for gestational diabetes in the second trimester and checks growth with a scan in the third. It also decides where you go in an emergency. At KKH, if you are 22 weeks or more you go to the Delivery Suite, and under 22 weeks to the Urgent O&G Centre.

Later scans compare your baby's size with the week count. Cleveland Clinic says a baby whose estimated weight is below the 10th percentile for its gestational age, lighter than 9 in 10 babies of the same age, is described as having intrauterine growth restriction (IUGR). It adds that measuring small does not always mean IUGR, and that most babies who measure small are born small but healthy. Read more under low birth weight. You can see what your current week looks like in our week-by-week pages, such as 22 weeks pregnant.

Frequently asked questions

What is gestational age in simple terms?

How many weeks and days pregnant you are, counted from the first day of your last period rather than from the day you conceived.

Is gestational age the same as my baby's age?

Not quite. It runs about two weeks ahead of the baby's age from conception, because the count starts at your last period, and Cleveland Clinic says it measures the pregnancy itself.

What does it mean if my baby is measuring small for my weeks?

Often nothing serious. Cleveland Clinic says babies grow at different rates and most who measure small are born small but healthy. If the estimated weight falls below the 10th percentile for gestational age, Cleveland Clinic says care then includes frequent monitoring and testing of the baby.

At what gestational age is a baby full term?

In Singapore you will hear 37 weeks: HealthHub calls a baby full term from then. ACOG's labels reserve full term for 39 weeks to 40 weeks 6 days and call 37 to 38 weeks early term.

Related terms

• LMP (Last Menstrual Period): The first day of your last period before pregnancy. Your week count and due date are worked out from it.
• EDD (Estimated Due Date): The date your baby is expected: 40 weeks from the first day of your last period, checked by a dating scan.
• Trimester: One of the three stages of pregnancy, each about three months long.
• Preterm (Premature) Birth: A baby born alive before 37 completed weeks of pregnancy. In Singapore, about 8.5% of babies arrive this early.
• Low Birth Weight: A birth weight under 2.5 kg, from being born early, growing slowly in the womb, or both.

See every term in the glossary