What is gestational age and how is it different from fetal age?

Gestational age (GA) is counted from the first day of the last menstrual period (LMP) and is the universal clinical standard. Fetal age (or embryonic age) counts from conception — roughly two weeks later. A baby reported as "12 weeks gestational age" is therefore about 10 weeks post-conception. All clinical paperwork, ultrasound reports, and due-date estimates use gestational age.

How is gestational age calculated from LMP?

Using Naegele's Rule (the ACOG and NICE standard): EDD = LMP + 280 days. Today's GA is (today minus LMP) expressed as whole weeks plus remaining days. For example, if your LMP was 70 days ago you are 10 weeks 0 days pregnant (10 x 7 = 70). The formula assumes a 28-day cycle with ovulation on day 14; for cycles that differ, an early ultrasound gives a more accurate EDD.

What if I do not know my LMP?

Use the ultrasound dating mode. In the first trimester a crown-rump length (CRL) measurement dates the pregnancy to within plus or minus 5 days — more accurate than LMP in most cases. In the second trimester, biparietal diameter (BPD) or head circumference (HC) is used, though accuracy widens to plus or minus 10 days. Enter the scan date and the GA your sonographer measured; the calculator works back to an LMP-equivalent automatically.

When does my trimester change?

The three trimesters follow ACOG and NICE convention. The first trimester runs from 0 weeks 0 days through 13 weeks 6 days (days 0 to 97 from LMP). The second trimester covers 14 weeks 0 days through 27 weeks 6 days (days 98 to 195). The third trimester begins at 28 weeks 0 days (day 196) and continues to birth. Some older texts define trimesters slightly differently, but this is the current clinical standard in the UK and US.

How accurate is the estimated due date?

Only about 4-5% of babies arrive on the exact EDD. The EDD is a statistical midpoint of a normal distribution roughly plus or minus 2 weeks wide. An ultrasound CRL in weeks 7-10 is the most accurate single predictor, typically accurate to plus or minus 3-5 days. LMP-based dating assumes a perfect 28-day cycle and is less reliable if cycles are irregular or the period before pregnancy was unusual. ACOG recommends keeping the EDD set by the earliest reliable ultrasound once it differs from LMP by more than 5 days (T1) or 10 days (T2).

What is the difference between early term, full term, and post-term?

ACOG (2013 revised definitions) splits "term" into four bands: early term (37 weeks 0 days to 38 weeks 6 days), full term (39 weeks 0 days to 40 weeks 6 days), late term (41 weeks 0 days to 41 weeks 6 days), and post-term (42 weeks and beyond). Outcomes are best for babies born in the full-term window; early-term births carry slightly higher risk of respiratory and feeding difficulties even though they are not technically preterm.

Gestational Age Calculator

Gestational age is the universal clinical measure of how far along a pregnancy is, counted in weeks and days from the first day of the last menstrual period (LMP). It is the number that appears on every ultrasound report, every antenatal chart, and every clinical guideline — from when to offer the nuchal translucency scan to when to consider induction. Because it counts from the LMP rather than from conception, gestational age is always about two weeks ahead of the embryo’s true biological age.

This calculator accepts three different starting points so it works no matter what information you have. If you know your LMP it applies Naegele’s Rule directly. If you know your conception date or IVF transfer date it converts backward by 14 days. If you have an ultrasound report it back-calculates from the scan date and the GA measured at that scan — the most accurate method when cycles are irregular or the LMP is uncertain.

How it works

Naegele’s Rule (the standard endorsed by ACOG, NICE, and the WHO) states:

EDD = LMP + 280 days

where 280 days equals exactly 40 weeks. Today’s gestational age is simply:

GA = today minus LMP, expressed as whole weeks W plus remaining days D.

For example, if your LMP was 98 days ago: 98 divided by 7 gives 14 weeks exactly — the first day of the second trimester.

Trimester boundaries follow ACOG and NICE convention:

First trimester: 0 weeks 0 days through 13 weeks 6 days (days 0–97)
Second trimester: 14 weeks 0 days through 27 weeks 6 days (days 98–195)
Third trimester: 28 weeks 0 days onward (days 196+)

Ultrasound dating mode computes a “LMP-equivalent” from the scan date and the measured GA: LMP-equivalent = scan date minus (measured GA in days). From that point the calculation is identical. ACOG recommends using the ultrasound EDD rather than the LMP EDD when the two differ by more than 5 days in the first trimester or more than 10 days in the second.

Conception mode subtracts 14 days from the conception or IVF transfer date (reflecting the typical 14-day follicular phase of a 28-day cycle) to obtain the LMP-equivalent.

Worked example

Suppose today is 1 June 2026 and your LMP was 3 March 2026:

Days elapsed: 90 days
Gestational age: 12 weeks 6 days (12 x 7 = 84; 90 - 84 = 6)
Trimester: First (day 90 is still within days 0–97)
EDD: 3 March 2026 + 280 days = 7 December 2026
Days remaining: 7 December minus 1 June = 189 days
Progress: (90 / 280) x 100 = 32.1% through the 40-week journey
Next milestone: end of first trimester in 7 days (day 97 = 13w6d)

If instead you enter an ultrasound from 15 April 2026 reporting 9 weeks 2 days (= 65 days), the calculator derives LMP-equivalent = 15 April minus 65 days = 9 February 2026, giving an EDD of 17 November 2026 — 20 days earlier than the LMP-based date, which a clinician would adopt because the T1 scan is more reliable.

Key milestones shown

The calculator highlights the current stage and next upcoming milestone from a clinically grounded timeline: heartbeat detectable (around week 6), end of the embryonic period (week 8), nuchal translucency screening window (weeks 11–13), anatomy scan (weeks 18–21), the viability threshold at 22 weeks, the ACOG “full term” window (39–40 weeks), and the post-term boundary at 42 weeks. Milestone data is sourced from Williams Obstetrics (25th ed.) and ACOG Practice Bulletins.

Educational use only — not medical advice. Gestational age calculations are estimates. Your clinician’s dating scan takes precedence. Always discuss your results with a qualified healthcare provider.

All calculations run entirely in your browser. No dates, no personal health information, and no results are ever transmitted anywhere.