Gestational Age Calculator (GA, Weeks + Days)
Compute the gestational age (GA) of a pregnancy in the clinical format weeks + days (e.g. 15 2/7). Input your last menstrual period (LMP) and the calculator returns your exact GA today, current trimester (ACOG definition), and the corresponding estimated due date (EDD). Intended for both expectant parents and health-care workers who need precise dating.
Gestational age is the time elapsed since the first day of your last menstrual period (LMP), reported in completed weeks and days (for example, 24w3d). It is counted from LMP, not conception, so it runs about two weeks ahead of fetal age. Full term is 40 weeks under ACOG dating. A first-trimester crown-rump-length ultrasound confirms it most accurately.
When to use this calculator
- A healthcare worker needs to record the GA in the mother's chart to schedule tests.
- You want to know if you're within the window for a specific screening (NT scan, quad screen, anatomy scan).
- You had an early ultrasound and want to cross-check CRL-based dating against LMP-based.
- You're tracking embryo development week by week using obstetric reference charts.
- You need to convert "months pregnant" to "weeks + days" for official forms.
Example: LMP January 15, 2026, today April 22
- LMP: January 15, 2026.
- Today: April 22, 2026.
- Days elapsed: 97 days.
- GA in weeks + days: 97 / 7 = 13 weeks + 6 days (written as 13 6/7).
- Trimester: still 1st (2nd starts at 14 0/7).
- EDD: LMP + 280 = October 22, 2026.
How it works
2 min readWhat exactly is gestational age?
Gestational age (GA) is the time elapsed since the first day of the last menstrual period (LMP), measured in weeks + days (0 0/7 … 42+ 0/7). It's distinct from embryonic age (which counts from conception/ovulation, ≈2 weeks later) and from fetal age (which typically equals embryonic age + 2 weeks).
Convention: when a chart says 15 3/7, the pregnancy is 15 complete weeks + 3 additional days. /7 is always the denominator.
How clinicians date a pregnancy
ACOG (2017) ranks the dating methods by accuracy:
1. First-trimester ultrasound (≤13 6/7) with CRL measurement — most accurate (±5 to 7 days).
2. LMP when the LMP date is certain and cycles are regular (28 days).
3. Second-trimester ultrasound (14 0/7 – 21 6/7) — ±7 to 10 days.
4. Third-trimester ultrasound — ±2 to 3 weeks (least reliable).
This calculator uses method 2 (LMP-based dating). If your early ultrasound dates the pregnancy >7 days differently than LMP (in T1), obstetricians redate to the ultrasound. See ACOG's redating table below.
ACOG redating thresholds
| Ultrasound window | Redate if discrepancy > |
|---|---|
| ≤ 8 6/7 | 5 days |
| 9 0/7 – 13 6/7 | 7 days |
| 14 0/7 – 15 6/7 | 7 days |
| 16 0/7 – 21 6/7 | 10 days |
| 22 0/7 – 27 6/7 | 14 days |
| ≥ 28 0/7 | 21 days |
Screening windows that depend on exact GA
Missing a window by a day can mean the test must be rescheduled or replaced. That's why GA is recorded in weeks + days, not just weeks.
Converting GA to months (for questions like "how many months pregnant?")
40 weeks / 9 months is approximate. Using lunar months of 4 weeks:
| GA | Months |
|---|---|
| 0 0/7 – 4 6/7 | 1 month |
| 5 0/7 – 8 6/7 | 2 months |
| 9 0/7 – 13 6/7 | 3 months (end of T1) |
| 14 0/7 – 17 6/7 | 4 months |
| 18 0/7 – 22 6/7 | 5 months |
| 23 0/7 – 27 6/7 | 6 months (end of T2) |
| 28 0/7 – 31 6/7 | 7 months |
| 32 0/7 – 35 6/7 | 8 months |
| 36 0/7 – 40 0/7 | 9 months |
When LMP-based dating fails
In any of these, ask for a first-trimester dating scan. It's the single most accurate tool in obstetrics.
Sources
Frequently asked questions
What does gestational age of 28 3/7 mean?
28 weeks + 3 days since the first day of the last menstrual period. The first number is complete weeks, the slash-7 is additional days (0–6). At 28 3/7 you are just entering the third trimester.
How is gestational age different from fetal age?
Gestational age counts from the first day of LMP (conception happens around week 2). Fetal age (or embryonic age) counts from conception, so it's typically 2 weeks less than GA. GA is the standard in medicine; fetal age is more often used in embryology texts.
How does a CRL ultrasound calculate gestational age?
The crown-rump length (CRL) is measured in millimeters on a first-trimester ultrasound. Using the Robinson-Fleming formula, the scanner computes GA as GA (days) = 8.052 × √CRL + 23.73. This method is accurate to ±5 days in the first trimester and is preferred over LMP when they disagree by more than 7 days (ACOG).
What is "menstrual age" vs "gestational age"?
They're the same thing. Both count from the first day of the last menstrual period. Menstrual age is the older term; gestational age is the modern standard.
Why do doctors count from LMP instead of from conception?
Because the exact day of conception is usually unknown, while LMP is a self-reported date that most women remember. Adding 14 days (typical ovulation in a 28-day cycle) to LMP would also work, but LMP is used directly for consistency across all pregnancies.
What if my cycle is 35 days, not 28?
LMP-based GA will be inaccurate by a few days because ovulation in a 35-day cycle is around day 21, not day 14. Request a first-trimester ultrasound for accurate dating. The OB may adjust your EDD by 5–7 days.
Can this calculator be used after an IVF cycle?
For IVF, dating is even simpler: the transfer date is known exactly. If you had a 5-day embryo (blastocyst) transfer, your LMP equivalent is transfer date − 19 days (14 for ovulation + 5 for blastocyst age). Enter that synthetic LMP to get the correct GA.
What's considered "full-term" in weeks?
ACOG defines full-term as 39 0/7 – 40 6/7 weeks. Early term is 37 0/7 – 38 6/7, late term is 41 0/7 – 41 6/7, and post-term is 42+. Preterm is anything before 37 0/7.