Gestational age calculator

Two weeks pregnant before you conceived

The first thing to understand about gestational age is that it is not the age of the pregnancy. It is the number of days since the first day of your last period, which is roughly two weeks before conception happened. At six weeks pregnant, an embryo has existed for about four weeks. At twelve weeks, about ten.

This is bookkeeping rather than biology. Gestational age is counted this way because the first day of a period is a date most people can name, whereas the day of conception usually is not. Every professional body uses the same convention, so the numbers travel: a “12-week scan” means the same thing in Manchester, Melbourne and Minneapolis. But it does mean the figure you are given will always run about a fortnight ahead of the age of the pregnancy itself, and nobody tends to mention that out loud.

Weeks, days, and the week you are “in”

Maternity notes write gestational age as weeks+days: 18+2 means eighteen completed weeks and two days. The plus sign is not an approximation. It is exact, and it is what your midwife will use.

The confusion comes from the difference between the weeks you have completed and the week you are in. At 18+2 you have completed 18 weeks and you are in your 19th. Both statements are true and they use different numbers, which is why a friend, an app and a midwife can all give you different answers on the same afternoon. This calculator shows both, deliberately.

Trimesters are a convention, not a boundary

The first trimester runs from 0+0 to 13+6, the second from 14+0 to 27+6, and the third from 28+0 to birth. The NHS week-by-week guide groups its pages the same way — weeks 4 to 12, weeks 13 to 27, weeks 28 to 41.

You will occasionally see the first trimester given as ending at 12 weeks instead of 14. Neither is wrong; they are different divisions of the same 40 weeks, and nothing happens to a pregnancy at midnight on the boundary. What does change is what is offered and when, which is a more useful thing to know than which third you are in.

Which date should you start from?

If you have had a dating scan, use the due date it gave you. Early ultrasound is the most accurate way to date a pregnancy because early embryos grow at a remarkably uniform rate — before about 14 weeks, size really does tell you age. The NHS offers this scan between 11 and 14 weeks, and it exists precisely to “check how many weeks pregnant you are and work out your due date”.

If you have not had the scan yet, the first day of your last period is the next best thing, adjusted for your cycle length. ACOG names the weakness of the unadjusted rule directly: it “does not account for inaccurate recall of the LMP, irregularities in cycle length, or variability in the timing of ovulation”. If your cycles run to 34 days you probably ovulate around day 20, six days later than Naegele assumed, and you are six days less far along than the rule says.

A known conception or ovulation date is more precise still, and an IVF date is the most precise there is, because the day of fertilisation is logged rather than inferred.

What happens around now

The most useful thing a gestational age gives you is not the number. It is knowing what the number means for the next few weeks, and the NHS antenatal schedule is specific about that.

The booking appointment with a midwife happens between 8 and 12 weeks and takes about an hour. The dating scan falls between 11 and 14 weeks, and screening for Down's, Edwards' and Patau's syndromes is offered at the same appointment. There is a 16-week appointment to go through screening results. The 20-week screening scan happens between 18 and 21 weeks. Testing for gestational diabetes is offered between 24 and 28 weeks if you are at higher risk. From 28 weeks the appointments come closer together — 28, 31 (first pregnancies), 34, 36, 38, 40 and 41 weeks — and from 36 weeks your midwife feels your bump at every visit to check which way up your baby is lying.

At 41 weeks the conversation changes: the NHS says “induction will be offered if you do not go into labour naturally by 41 weeks, as there will be a higher risk of stillbirth or problems for the baby”.

Term is not one thing

The word “term” covers five weeks and four separate bands, and the distinction matters. ACOG defines early term as 37+0 to 38+6, full term as 39+0 to 40+6, late term as 41+0 to 41+6, and post term from 42+0. Babies born at 39 weeks do measurably better than those born at 37, which is why planned birth without a medical reason is not offered before 39 weeks.

The NHS puts the range plainly on its own due date calculator: “Pregnancy normally lasts from 37 weeks to 42 weeks from the first day of your last period.” That is a five-week window, and your due date sits in the middle of it rather than at the end of it.

What the percentage does and does not mean

Showing how much of the 280 days is behind you is a reasonable way to orient yourself, and a terrible way to predict anything. Jukic and colleagues, writing in Human Reproduction in 2013, found that only about 4% of women deliver at 280 days and only about 70% deliver within 10 days of the estimated date. In their own data, based on pregnancies where ovulation was measured directly, term gestation still varied by 37 days.

So: 90% complete does not mean ten per cent of the time is left. It means you are ten per cent of the way from here to the middle of a five-week window.

What this cannot do

This is arithmetic on a date you gave it. It cannot see your pregnancy, it cannot detect twins — who are dated the same way but rarely carried to 40 weeks — and it cannot tell you whether an irregular cycle means you ovulated late this month. ACOG considers a pregnancy without a dating ultrasound before 22 weeks to be “suboptimally dated”.

If something has changed — bleeding, pain, a change in your baby's movements — the number on this page is not the thing to look at. Contact your midwife or maternity unit, at any hour, on any day.