Gestational Age Calculator

1. What gestational age is, and how to read 24+3

Gestational age is measured in completed weeks plus days since the first day of the last menstrual period, not since conception. It is written with a plus sign: 24+3 means 24 completed weeks and 3 days, which is what appears on a maternity record, a scan report and every clinical note.

The counting starts at the LMP because that is the date most people can actually identify. Conception happens roughly two weeks later, around ovulation, so gestational age is conventionally about a fortnight ahead of the true age of the embryo — sometimes called fertilisation or conception age. Both are correct; they are answers to different questions, and confusing them accounts for most of the two-week discrepancies people run into.

The notation resets at each whole week. 24+6 is followed by 25+0, so "25 weeks" in conversation usually means the day the record reads 25+0, while "in the 25th week" means anywhere from 24+0 to 24+6. The calculator prints the exact weeks+days string so there is nothing to infer.

2. Naegele’s rule and the estimated due date

The due date comes from a formula published by Franz Naegele in the early 1800s and still in routine clinical use: add one year, subtract three months, add seven days to the first day of the last menstrual period. That is 280 days — exactly 40 weeks.

EDD = LMP + 280 days

Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is reliably longer or shorter, ovulation is correspondingly later or earlier, and the due date shifts by the difference. The cycle-length field applies that correction: a 33-day cycle moves the estimate five days later, a 24-day cycle moves it four days earlier. The gestational age itself does not change, because that is still counted from the LMP — only the endpoint moves.

It is worth being blunt about what the estimate is. Fewer than one in twenty births happen on the estimated due date; a term birth anywhere from three weeks before to two weeks after is entirely ordinary. The EDD is a reference point that clinical decisions are anchored to, not a prediction of a day.

3. Three ways in: LMP, conception date, or a known due date

The method selector accepts whichever date you actually have, and all three routes are reduced to the same LMP-equivalent day count internally, so they cannot disagree with each other:

• Last menstrual period. The clinical default. Enter the first day of bleeding, not the last.
• Conception or ovulation date. Useful if you tracked ovulation, used an LH test, or know the date of insemination or transfer. The calculator works back 14 days — adjusted for your cycle length — to the LMP equivalent.
• Known estimated due date. If a scan has already given you a date, enter it and the calculator runs the arithmetic backwards to give you gestational age and the implied LMP. This is the option to use after a dating scan, because it keeps every figure consistent with the date your clinic is working to.

Enter a due date from an early scan and the milestone dates, trimester boundaries and term classification all realign to it. That matters more than it sounds: if a scan has moved your dates, an LMP-based figure will be wrong by however much the scan moved them, for the rest of the pregnancy.

4. Trimesters, and what “full term” means since 2013

Trimesters are conventional divisions rather than biological boundaries. The calculator uses the common clinical split: first trimester to 13+6, second from 14+0 to 27+6, third from 28+0.

Term classification is more precise, and it changed. Until 2013 anything from 37 to 42 weeks was called "term", which obscured real differences in outcome across those five weeks. A joint ACOG and Society for Maternal-Fetal Medicine workgroup replaced the single label with five:

The practical consequence is that “full term” now begins at 39 weeks, not 37. Elective delivery in the early-term window is treated differently from delivery at 39 weeks as a direct result of this reclassification. The term tile above reports which band the current figure falls in, and the milestone list marks 39 weeks as its own entry rather than the due date alone.

5. When a scan beats the calendar

A first-trimester ultrasound is more accurate than LMP dating, and where the two disagree the scan normally wins. Crown-rump length before about 14 weeks predicts gestational age to within roughly five to seven days, because embryos grow at a very consistent rate that early. After the first trimester, biometry becomes progressively less reliable for dating as normal variation in growth widens.

This is why LMP dating is only ever provisional. It depends on remembering the date, on having a regular cycle, on ovulation falling where the formula assumes, and on the bleeding being a true period rather than spotting. Any one of those failing moves the estimate by days to weeks. Practice guidelines set specific thresholds for how large a discrepancy justifies redating, and applying them is a clinical decision, not an arithmetic one.

Assisted conception is the exception in the other direction: with IVF the fertilisation or transfer date is known exactly, so the due date is calculated from it and is the most reliable dating there is. Use your clinic's date with the known due date method rather than working from an LMP.

Nothing you enter leaves your browser. Pregnancy dates are among the most sensitive data anyone types into a website, and this page has no analytics on the input, no server round-trip and no storage — the arithmetic runs on your own device and the page works with the network switched off. After the birth, the baby age calculator converts gestational age at birth into corrected age for milestone tracking, and once your child is past that stage the child age calculator takes over with age in completed months and years.