Gestational Age Calculator
Find out exactly how far along you are — in weeks and days — using your last menstrual period, an ultrasound measurement, conception date, or IVF transfer date. Medically reviewed and based on ACOG dating guidelines.
Calculate Your Gestational Age
Choose the dating method that applies to you. All calculations use the same clinical standards your provider uses.
Enter a date above to see your baby’s approximate size this week.
What Is Gestational Age?
Gestational age is the standard measure of how far along a pregnancy is, expressed in completed weeks and days. Unlike “fetal age” (which counts from the actual date of conception), gestational age is counted from the first day of your last menstrual period (LMP) — roughly two weeks before conception actually occurs. This is the convention used by the American College of Obstetricians and Gynecologists (ACOG), the NHS, and virtually every clinician worldwide, which is why your due date, prenatal appointment schedule, and screening windows are all based on it.
How Gestational Age Is Calculated
This calculator uses three clinically recognized dating methods, matching what’s used in prenatal care:
- LMP-based dating (Naegele’s rule): Gestational age = today’s date minus the first day of your LMP. Your estimated due date = LMP + 280 days (40 weeks), adjusted for cycle length if it differs from 28 days.
- Ultrasound dating: Most accurate in the first trimester, particularly between 8–13 weeks via crown-rump length (CRL). The calculator adds the elapsed time since your scan to the gestational age recorded that day.
- Conception-based dating: If your conception date is known precisely (e.g., from ovulation tracking or a single intercourse date), gestational age = days since conception + 14 days.
- IVF transfer dating: The most precise method available, since the embryo’s exact age is known. A day-5 transfer is dated as 19 weeks pregnant + 5 days; a day-3 transfer as 19 weeks pregnant + 3 days at the time of transfer.
Which Method Is Most Accurate?
| Method | Typical Accuracy | Best Used When |
|---|---|---|
| First-trimester ultrasound (CRL) | ± 3–5 days | Scan performed 8–13 weeks |
| IVF transfer date | ± 1 day | Always, for IVF pregnancies |
| Known conception date | ± 1–3 days | Ovulation tracking, single cycle |
| LMP (regular cycles) | ± 5–7 days | Consistent 28-day cycle |
| Second-trimester ultrasound | ± 7–10 days | 14–27 weeks |
| Third-trimester ultrasound | ± 14–21 days | 28+ weeks (least reliable for dating) |
ACOG guidance is clear: once a due date is established, it should rarely be changed — and an early ultrasound generally takes precedence over LMP dating if the two differ by more than the thresholds above.
Gestational Age vs. Fetal Age — What’s the Difference?
This is one of the most common points of confusion in early pregnancy. Gestational age starts counting from the LMP, while fetal age (or “conceptional age”) starts from actual fertilization — about two weeks later. So at “6 weeks pregnant” (gestational age), the embryo itself is only about 4 weeks old developmentally. Every calculator, app, and prenatal chart you’ll encounter uses gestational age unless explicitly labeled otherwise.
Why Gestational Age Matters
- Scheduling prenatal care: Screening tests (NIPT, nuchal translucency, anatomy scan, glucose tolerance test) are all timed to specific gestational windows.
- Monitoring fetal growth: Measurements like estimated fetal weight are only meaningful when compared against the correct gestational age on a growth chart.
- Defining preterm vs. full-term birth: Birth before 37 weeks is preterm; 39–40 weeks 6 days is considered “full term.”
- Guiding labor decisions: Induction timing, especially for high-risk pregnancies, depends on accurate dating.
Explore More Pregnancy Tools and Guides
Once you know your gestational age, plan ahead with our full collection of free pregnancy calculators, from due date and ovulation to hCG and growth percentiles. For explanations of how dating works, browse the PregCalc blog.
Frequently Asked Questions
Medically reviewed by Dr. Ruqaiya Khan, MD, FACOG
Dr. Khan reviews all dating and gestational-age content on pregcalc.com for clinical accuracy against current ACOG and NHS guidelines. View full reviewer credentials
Sources
- American College of Obstetricians and Gynecologists (ACOG) — Methods for Estimating the Due Date, Committee Opinion No. 700.
- NHS — Your Pregnancy and Baby Guide: Calculating Your Due Date.
- Mayo Clinic — Fetal Development: The First Trimester / Second Trimester / Third Trimester.
Curious about your baby’s arrival date? Use our Due Date Calculator to convert your gestational age into an estimated due date, or review the ACOG committee opinion on methods for estimating due dates for the clinical basis behind this gestational age calculator.