What is gestational age?

Gestational age is how far along a pregnancy is, measured in weeks and days from the first day of the last menstrual period (LMP) — not from conception. A full-term pregnancy is 40 weeks (280 days) by this dating, even though fertilization happens about two weeks after the LMP.

How is gestational age calculated from the last menstrual period (LMP)?

Subtract the LMP date from today's date: the difference in days is the gestational age. The estimated due date (EDD) is the LMP plus 280 days — this is Naegele's rule, the standard clinicians use for regular 28-day cycles.

How do you calculate gestational age from the conception date?

Add 14 days to the time since conception: gestational age = (today − conception date) + 14 days. The due date is the conception date plus 266 days (38 weeks), because ovulation is assumed to occur on day 14 of the cycle.

How is gestational age estimated from an ultrasound (CRL)?

In the first trimester the crown-rump length (CRL) is measured on ultrasound and converted to gestational age with the Robinson formula: GA (days) = 8.052 × √(CRL in mm) + 23.73. The scan date minus that age gives an LMP-equivalent, and adding 280 days gives the due date.

How is gestational age calculated after IVF?

Add the embryo's age at transfer to the usual 14-day allowance: gestational age = (today − transfer date) + 14 + embryo age in days. That is +17 days for a day-3 cleavage transfer and +19 days for a day-5 blastocyst transfer (day 6: +20).

What is the difference between gestational age and fetal age?

Gestational age is counted from the last menstrual period; fetal age (also called embryonic or developmental age) is counted from conception and is about 14 days less. When a doctor says 12 weeks pregnant, the fetus is about 10 weeks old.

What are the trimesters of pregnancy?

The first trimester runs from week 1 through week 13 (days 0–97), the second from week 14 through week 27 (days 98–195), and the third from week 28 until delivery (day 196 onward).

Why does my ultrasound due date differ from my LMP due date?

LMP dating assumes ovulation on day 14, which is often wrong; first-trimester ultrasound (CRL) is more accurate. Clinicians generally re-date the pregnancy from the ultrasound when the two differ by more than 5–7 days in the first trimester — always confirm with your provider.

What does it mean if gestational age goes past 42 weeks?

Pregnancies reaching 42 weeks and beyond are called post-term. Most providers will not let a pregnancy continue far past the due date without extra monitoring or discussing induction, because risks rise after 41–42 weeks — talk to your clinician promptly.

Is this calculator medical advice?

No. This calculator gives educational estimates from standard dating formulas. It is not medical advice and cannot replace an ultrasound or your clinician's dating — confirm every result with your doctor or midwife.

Gestational Age Calculator

How many weeks pregnant are you right now? Enter your last period, conception date, ultrasound CRL, or IVF transfer date to get your current gestational age in weeks + days, your due date, trimester, countdown, and a visual 40-week timeline.

Educational estimate — not medical advice. This tool applies standard pregnancy-dating formulas. It cannot replace an ultrasound or your clinician’s dating. Confirm every result with your doctor or midwife.

Choose your starting point

First day of your last menstrual period

Day 1 of bleeding, not spotting. Standard dating assumes a 28-day cycle.

Calculate as of (defaults to today)

Move this date to see how far along you will be on a future date — e.g. a flight or appointment.

Scope boundary — which tool do you need?

This tool answers: how far along am I right now? — current gestational age in weeks + days, trimester, and countdown to the due date, from any of four starting points.

Due-date deep dive? For EDD-centered planning — milestones, baby-size comparison, and a calendar file — use our pregnancy due date calculator.
Fetal measurements? For weight percentile from ultrasound biometry (BPD, HC, AC, FL), use our fetal weight percentile calculator — it needs the gestational age this tool computes.
Not pregnant yet? For fertility timing, use our ovulation calculator; for when conception likely happened, our conception calculator.

How it is calculated

The four dating formulas

LMP (Naegele’s rule): GA = today − LMP · EDD = LMP + 280 days

Conception: GA = (today − conception) + 14 days · EDD = conception + 266 days

Ultrasound CRL (Robinson, 1975): GA at scan = 8.052 × √(CRL in mm) + 23.73 days · EDD = scan date + (280 − GA at scan)

IVF transfer: GA = (today − transfer) + 14 + embryo age · EDD = transfer + (266 − embryo age)

Embryo age is 3 days for a cleavage-stage transfer, 5 for a day-5 blastocyst, 6 for a day-6 blastocyst. All date math is done in whole UTC days, so timezones and daylight-saving changes cannot shift the result.

Trimester	Weeks	Days from LMP
First	1–13	0–97
Second	14–27	98–195
Third	28–delivery	196+

Key reference points: conception ≈ day 14 · end of 1st trimester day 97 (13w6d) · viability ≈ day 168 (24w) · full term day 259 (37w) · due date day 280 (40w). Only about 4–5% of babies arrive on the exact due date — most come within two weeks either side.

Frequently asked questions

Gestational age FAQ

Free tool by NRTechStudio

Gestational Age Calculator

Scope boundary — which tool do you need?

The four dating formulas

Gestational age FAQ

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