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Gestational Age

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What is Gestational Age?

Gestational age is the age of a pregnancy measured in weeks and days from the first day of the last menstrual period (LMP), not from the date of conception. A full-term pregnancy is 40 weeks of gestational age. This counting method starts about two weeks before fertilization actually occurs, because ovulation and conception happen roughly two weeks after the LMP begins.

All obstetric milestones are expressed in gestational weeks: viability is established around week 24, full term begins at week 37, and post-term is week 42 or later. A baby delivered at 40 weeks gestational age has been developing as an embryo or fetus for approximately 38 weeks - the two figures differ because gestational age includes the pre-conception phase of the menstrual cycle.

Gestational Age explained to a beginner

Imagine a race where the clock starts two minutes before the runners leave the starting line. The runners are not actually moving for those first two minutes, but the official race time includes them anyway. Gestational age works the same way - the pregnancy "clock" starts at your last period, not when conception happened. So when your doctor says you are 10 weeks pregnant, the baby has only existed for about 8 weeks.

When to use Gestational Age

Gestational age is the number your obstetrician, midwife, and every pregnancy app uses for scheduling screenings and interpreting growth measurements. Use it when reading ultrasound reports, scheduling the anatomy scan (weeks 18-22), understanding test windows, and tracking fetal size milestones. The pregnancy calculator on this page calculates gestational age from whichever input you provide.

Worked examples for Gestational Age

This table quickly gives you the overview you need to understand Gestational Age and its most important comparisons.

Gestational age	Approximate fetal age	Clinical milestone
6 weeks	4 weeks	Heartbeat detectable by transvaginal ultrasound
10-13 weeks	8-11 weeks	First trimester screening and nuchal translucency scan
20 weeks	18 weeks	Halfway point; anatomy scan window
24 weeks	22 weeks	Viability threshold
37 weeks	35 weeks	Full term begins
40 weeks	38 weeks	Estimated due date

Common pitfalls

Do not confuse gestational age with fetal age (also called embryonic age or fertilization age). Gestational age is always about 2 weeks more than fetal age. When a pregnancy website says "your baby at week 8," check whether it means gestational week 8 (fetal age ~6 weeks, embryo about the size of a raspberry) or fetal week 8 (gestational age ~10 weeks, about the size of a kumquat). The difference is a full trimester of development on some consumer apps.

Frequently asked questions about Gestational Age

Why does gestational age start at the last period and not conception?

Most people know when their last period started, but few know the exact date of conception. The LMP date is a reliable, observable event that can be recorded consistently. Counting from the LMP also means all providers use the same starting point, keeping screening timelines and viability thresholds standardized across all pregnancies worldwide.

Can gestational age be wrong if my cycles are irregular?

Yes. LMP-based gestational age assumes ovulation on day 14 of a 28-day cycle. If you have longer cycles (e.g., 35 days), ovulation happens around day 21, so LMP dating overestimates gestational age by about a week. A first-trimester ultrasound corrects for this by measuring the embryo directly, and ACOG recommends adjusting the due date if ultrasound differs by more than 7 days.

What is the difference between gestational age and fetal age?

Gestational age is measured from the LMP and includes the two weeks before conception. Fetal age (embryonic age) is measured from fertilization and is always about 2 weeks less than gestational age. Clinicians universally use gestational age; fetal age appears more often in embryology textbooks and some consumer pregnancy apps.

Test your knowledge

Quiz: how well do you know gestational age?

3 questions · ~1 min

1. A full-term pregnancy is defined as how many weeks of gestational age?

ℹ A full-term pregnancy is 40 weeks of gestational age, counted from the first day of the last menstrual period. "Full term" as a clinical label refers to 37-41 weeks. 38 weeks is the approximate fetal age at term - 2 weeks less than gestational age because gestational counting starts before conception.

2. If a woman is 12 weeks gestational age, approximately how long has the fetus been developing?

ℹ Gestational age is approximately 2 weeks more than fetal (developmental) age. At 12 weeks gestational age, the embryo/fetus has been developing for about 10 weeks, since gestational counting starts at the LMP - about 2 weeks before ovulation and conception actually occur.

3. Why is gestational age measured from the LMP rather than from conception?

ℹ The LMP is used because it is a known, recorded date - most people can recall or check when their last period started. The exact date of conception is rarely known from natural intercourse. Using a consistent, observable starting point allows all providers worldwide to use the same timeline for screening windows and viability thresholds.