1What Is Gestational Age?
Gestational age (GA) is the standard clinical measure of how far along a pregnancy has progressed. It is expressed in weeks and days — for example, "10 weeks and 3 days" or commonly written as "10+3." A full-term pregnancy spans 40 weeks from the start of the last menstrual period.
Importantly, gestational age is not counted from the moment of conception. Because ovulation and fertilisation typically occur about two weeks after the last period begins, gestational age is approximately two weeks longer than the embryonic (fetal) age. This convention is used worldwide by obstetricians, midwives, and ultrasound technicians to keep a single consistent standard.
Accurate gestational dating is critical because it determines when key prenatal tests are scheduled, guides growth monitoring, informs decisions about induction of labour, and classifies deliveries as preterm (<37 weeks), term (37–42 weeks), or post-term (>42 weeks).
From LMP
The most common method. Count weeks from the first day of your last period. Assumes a 28-day cycle.
From Ultrasound
The most accurate method, especially in the first trimester. CRL measurement at 6–13 weeks.
From Due Date
Work backwards from your EDD. Gestational age = 40 weeks minus time remaining until delivery.
Fetal Age vs. GA
Fetal (embryonic) age = gestational age minus 2 weeks. GA is always the clinical standard.
2How Is Gestational Age Calculated?
There are three widely accepted methods for calculating gestational age. Each has specific indications and accuracy ranges:
| Method | Formula | Accuracy | Best Used When |
|---|---|---|---|
| Last Menstrual Period (LMP) | GA = (Today − LMP) ÷ 7EDD = LMP + 280 days | ±2 weeks | Regular 28-day cycles, LMP known |
| Ultrasound — Crown-Rump Length (CRL) | CRL 1–84 mm → GA tables | ±5–7 days (1st trimester) | 6–13 weeks; irregular cycles |
| Ultrasound — Biometry (BPD, FL, HC) | Hadlock nomograms | ±2–3 weeks (late 2nd/3rd trimester) | After 14 weeks if CRL missed |
| Estimated Due Date (EDD) | GA = 40 weeks − Days to EDD ÷ 7 | Depends on EDD source | When EDD already established |
Naegele's Rule — The LMP Formula Explained
The most widely used method is Naegele's Rule, developed in 1812 and still the global obstetric standard. It adds 280 days (40 weeks) to the first day of the LMP to estimate the due date:
EDD = LMP + 280 days — or equivalently — EDD = LMP + 9 months + 7 days
The rule assumes a 28-day menstrual cycle with ovulation on day 14. For cycles shorter or longer than 28 days, the EDD is adjusted by the difference: a 32-day cycle shifts the EDD four days later, while a 24-day cycle shifts it four days earlier. Our calculator applies this correction automatically.
When Ultrasound Overrides LMP Dating
ACOG (American College of Obstetricians and Gynecologists) recommends that ultrasound dating in the first trimester should replace LMP dating if the difference is more than 7 days (CRL <9 weeks) or more than 10 days (CRL 9.0–13.9 weeks). First-trimester ultrasound remains the gold standard when LMP is unknown, uncertain, or when cycle length is irregular.
3Pregnancy Trimesters by Week
Pregnancy is divided into three trimesters. Each marks a distinct phase of fetal development and dictates which prenatal screenings, scans, and tests are appropriate:
Most critical period of organ formation.
- Week 6: Fetal heartbeat detectable
- Week 8: All major organs forming
- Week 10: Nuchal translucency scan
- Week 11–13: NIPT / First-trimester screening
Growth and maturation phase.
- Week 16: Quadruple marker screen
- Week 18–22: Anatomy ultrasound
- Week 20: Fetal movements felt
- Week 24: Viability threshold
Rapid weight gain and preparation for birth.
- Week 28: GDM screen & Rh immunoglobulin
- Week 32–34: Growth scan
- Week 36: GBS screening
- Week 40: Full-term delivery
Preterm, Term, and Post-Term Definitions
The World Health Organization and ACOG classify gestational age at delivery as follows:
- Extremely preterm: <28 weeks
- Very preterm: 28 to <32 weeks
- Moderate to late preterm: 32 to <37 weeks
- Early term: 37+0 to 38+6 weeks
- Full term: 39+0 to 40+6 weeks
- Late term: 41+0 to 41+6 weeks
- Post-term: 42+0 weeks and beyond
4Medical References & Sources
This calculator and content are based on established obstetric guidelines and peer-reviewed literature:
Last medical review: May 2025 · Content reviewed by RxMedCalc Clinical Editorial Team.
5Frequently Asked Questions
Gestational age tracks pregnancy progress in weeks and days from the last menstrual period, forming the baseline for all obstetric monitoring and milestone tracking.
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Medical disclaimer: This calculator is for educational and clinical decision-support purposes only. It does not replace clinical judgment or specialist consultation. RxMedCalc is not liable for clinical decisions made solely on this tool.