What is gestational age?
Gestational age (GA) is how far along a pregnancy is, measured in weeks and days from the first day of the last menstrual period. It is the universal language of obstetrics: scan reports, lab forms, discharge summaries and paediatric notes all use it. A newborn’s GA at birth guides decisions about feeding, monitoring and vaccinations.
Unlike a simple due date calculator, this tool lets you pick any date. Want to know how far along you’ll be on your sister’s wedding day, or what your GA was on the day of a blood test? Change the “Calculate for this date” field.
Gestational age vs fetal age
Fetal age (also called conceptional or embryonic age) counts from conception, so it is about two weeks less than gestational age. Embryology textbooks often use fetal age, which is why you may read that the heart starts beating at “3 weeks” in one source and “5 weeks” in another – they are describing the same moment.
When you need GA on a specific date
- Booking scans: the NT scan is only possible between 11w 0d and 13w 6d; the anomaly scan is usually done at 18–22 weeks.
- Blood tests: first-trimester screening (double marker) and second-trimester screening (quadruple marker) use GA-specific reference ranges.
- Travel: many airlines ask for a fitness-to-fly letter after 28 weeks and restrict travel after 32–36 weeks.
- Leave planning: under the Maternity Benefit (Amendment) Act 2017, eligible women in India can take up to 26 weeks of paid leave, of which up to 8 weeks can be taken before the expected delivery date.
When ultrasound overrides your LMP
If your LMP date and your earliest scan disagree, doctors follow published thresholds before changing your dates. The earlier the scan, the smaller the difference needed:
| GA at scan (by LMP) | Re-date if difference is more than |
|---|---|
| Up to 8w 6d | 5 days |
| 9w 0d – 13w 6d | 7 days |
| 14w 0d – 15w 6d | 7 days |
| 16w 0d – 21w 6d | 10 days |
| 22w 0d – 27w 6d | 14 days |
| 28w 0d and later | 21 days |
If your pregnancy has been re-dated, use the Ultrasound tab with your first-trimester scan result, or the Due date tab with the date your doctor gave you.
Frequently asked questions
How is gestational age calculated?
Gestational age is the number of days between the first day of your last period and the date in question, written as completed weeks plus days. For example, 89 days is 12 weeks 5 days.
Is gestational age the same as how many weeks pregnant I am?
Yes. When people say they are ‘20 weeks pregnant’ they are describing gestational age.
How accurate is gestational age by ultrasound?
A crown-rump length measurement in the first trimester is accurate to within about 5–7 days. Accuracy falls later in pregnancy because babies grow at different rates.
What gestational age is considered viable?
In modern neonatal units, survival becomes possible from about 23–24 weeks, and outcomes improve sharply with every additional week. 24 weeks is commonly used as the viability milestone.
Can I calculate gestational age at birth?
Yes. Enter your LMP or due date and set the ‘Calculate for this date’ field to the birth date.
Medical disclaimer: The results and information on this page are estimates for general education only. They are not a diagnosis and do not replace advice from your gynaecologist, obstetrician or midwife. If you have pain, bleeding, reduced baby movements or feel unwell, contact your doctor or call 108 immediately. Read the full disclaimer.
Sources & further reading
- American College of Obstetricians and Gynecologists (ACOG). Committee Opinion No. 700: Methods for Estimating the Due Date. acog.org
- ACOG. Committee Opinion No. 579: Definition of Term Pregnancy.
- World Health Organization. WHO recommendations on antenatal care for a positive pregnancy experience (2016). who.int