Pregnancy Calculator
Work out your due date from whichever date you have — last period, scan, conception or IVF transfer — and see the whole timeline week by week.
Every figure here is an estimate. Fewer than one baby in twenty arrives on the estimated due date, and most births fall within a fortnight either side of it.
| Week | Dates | Trimester | Milestone |
|---|
Dates run from the first day of your last period, which is how pregnancy is counted clinically — so the first fortnight precedes conception. Milestones are typical averages, not a schedule any individual pregnancy has to follow. These figures describe a single pregnancy; twins and other multiples run differently throughout.
| Based on | Due date is |
|---|---|
| Last period | +280 days, adjusted for cycle length |
| Conception date | +266 days |
| IVF transfer | +266 days, less the embryo's age |
| Ultrasound | scan date + the time still to run |
| Due date | taken as given |
A dating ultrasound in the first trimester is the most accurate of these. Last-period dating assumes ovulation on day 14, which is why the cycle-length field matters if yours is not 28 days.
| Trimester | Weeks |
|---|---|
| First | 1 – 12 |
| Second | 13 – 27 |
| Third | 28 – 42 |
Full term runs from week 37. A birth before 37 weeks is preterm and after 42 is post-term, both of which are managed rather than simply waited out.
This page is educational and nothing on it is medical advice or a substitute for antenatal care. Dates it produces are estimates. Where they disagree with the dates your midwife, obstetrician or doctor has given you, theirs are the ones to go by — they have your scans and your history, and this page has neither.
How pregnancy is counted
Pregnancy is counted from the first day of your last period, not from conception. That sounds wrong the first time you hear it, and it has a practical reason: almost everyone can name the day their last period started, while almost nobody can name the day they conceived.
The consequence is that the count runs about two weeks ahead of the biology. Weeks 1 and 2 happen before there is an embryo at all. Conception falls in week 3, and by the time a test reads positive you are usually called four weeks pregnant despite the pregnancy being around a fortnight old. Every clinic, scan report and app uses this convention, so it is worth getting used to rather than mentally correcting.
A full term is 40 weeks from that first day — 280 days. The table on this page runs to 42 weeks, because that is the outer edge of what is considered normal rather than post-term.
The five ways to date a pregnancy
The calculator accepts whichever piece of information you actually have, and every route ends at the same place: an estimated due date, from which the whole timeline is built.
Conception date · due = conception + 266 days
IVF transfer · due = transfer + 266 days − the embryo's age at transfer
Ultrasound · due = scan date + however much of the 280 days was still to run
Due date · taken as given, and the timeline built backwards
The cycle adjustment is the part most calculators skip. Standard dating assumes you ovulate on day 14, which holds for a 28-day cycle and drifts for anything else. On a 35-day cycle ovulation usually comes about a week later, so the due date moves back about a week too — a real difference when you are being told whether you are early or late.
The IVF route is the most precise available, because fertilisation happened on a known date in a laboratory. Subtracting the embryo's age matters: a day-5 blastocyst had already been developing for five days when it was transferred, and dating that ignored it would run five days late.
What a due date can and cannot tell you
A due date is a midpoint, not an appointment. Fewer than one baby in twenty is born on it. Roughly 60% arrive within a week either side and close to 90% within two — which means the honest way to read your date is as the centre of a month-wide window.
Accuracy also depends on where the date came from. A first-trimester ultrasound is the best of the five, because embryos grow at almost the same rate early on, so size genuinely indicates age. That stops being true later: by the third trimester babies differ enough in size that a scan tells you how big this baby is, not how old. This is why a clinic will re-date a pregnancy from an early scan but not a late one.
If an early scan and your last period disagree by more than about five to seven days, clinicians generally go with the scan. That is not the calculator being wrong — it is better information arriving.
Trimesters and the week-by-week table
The three trimesters used here are weeks 1 to 12, 13 to 27, and 28 onward. Boundaries vary a little between sources, and nothing physiological happens the moment one ends; they are a convenient way to talk about phases that do genuinely differ.
The table marks the week you are in now and flags the milestones that are widely used as landmarks — a heartbeat visible on a scan around week 6, the sharp drop in miscarriage risk around week 13, movements becoming noticeable somewhere in the late teens, viability with intensive care around week 23, and full term from 37.
Treat those as averages with wide spread around them, particularly the movement one: a first pregnancy often feels movement several weeks later than a second, and both are entirely normal. The table describes a typical pregnancy, not a schedule yours is failing to keep.
Reading the size figures
The length and weight shown for your week are population averages, and the spread around them is wide. Two healthy babies at the same gestational age can differ by a pound or more near term.
One quirk worth knowing: measured length jumps sharply around week 21. That is not a growth spurt but a change of ruler. Before that, babies are measured crown to rump, because the legs are curled and cannot be measured reliably; from then on the measurement runs crown to heel. The jump in the numbers is the measurement changing, not the baby.
Beyond week 40 no separate figures are published, so the calculator holds the week-40 values and says so rather than inventing a number.
What this calculator does not cover
- Twins and higher multiples. Different growth, different dating, different expected arrival. None of this applies.
- Your particular pregnancy. These are population averages. Your care team has your scans and your history; a web page has neither.
- Anything to do with risk. This tool counts days. It cannot tell you whether anything is going well or badly, and it is not a screening test.
- Irregular or absent cycles. If your cycles vary a lot, last-period dating is weak from the start. An early scan is worth far more.
- Re-dating. If a scan has already moved your date, use that date here rather than recalculating from your last period.
If anything about your dates or your pregnancy concerns you, contact your midwife, obstetrician or doctor — that is what they are there for, and it is never too small a question.
Frequently asked questions
How accurate is a due date?
It is an estimate, and a fairly loose one. Fewer than one baby in twenty arrives on the estimated date. Around 60% arrive within a week either side and close to 90% within two weeks. A date is useful for planning care and judging whether labour is early or late — it was never meant to predict a particular day.
Which method gives the most reliable date?
An ultrasound in the first trimester. Early fetal growth varies little between pregnancies, so a scan before about 14 weeks dates a pregnancy more tightly than anything else. Last-period dating is the usual starting point but assumes ovulation on day 14, which is why an irregular or long cycle can shift it by several days.
Why does pregnancy start counting two weeks before conception?
Because it is counted from the first day of your last period, which is a date most people can identify, whereas conception usually cannot be pinned down. The convention means you are already called "four weeks pregnant" about a fortnight after conception. Weeks 1 and 2 of the count precede conception entirely.
How is the IVF due date different?
It is the most precise of the five, because the date of fertilisation is known rather than estimated. The calculator counts 266 days from conception and then subtracts the embryo's age at transfer — three, five or six days — since that much development had already happened before the transfer took place.
What does my cycle length change?
Where ovulation is assumed to fall. The standard calculation assumes a 28-day cycle with ovulation on day 14. On a 35-day cycle ovulation typically comes about a week later, so the due date moves back by roughly the same amount. Setting your average length makes last-period dating meaningfully better if your cycles are not 28 days.
Can my due date change during pregnancy?
Yes, and it commonly does. If an early scan disagrees with last-period dating by more than about five to seven days, clinicians usually adopt the scan date. Later scans are not used to re-date a pregnancy, because by then growth varies enough between babies that size no longer tells you age reliably.
What counts as full term?
From 37 weeks. Before that is preterm; from 42 weeks is post-term. Within the term window the distinctions still matter clinically — 39 to 40 weeks is generally considered the best window for the baby — which is one reason induction is not usually offered simply for reaching the due date.
Do these figures apply to twins?
No. Everything on this page describes a single pregnancy. Twins and higher multiples grow on a different curve, are dated and monitored differently, and very often arrive well before 40 weeks. If you are carrying more than one baby, use the dates your care team gives you rather than these.
This pregnancy calculator is an educational tool. It does not provide medical advice, diagnose anything, monitor a pregnancy, or replace antenatal care, and every date and measurement on it is a population estimate rather than a statement about you or your baby. It describes single pregnancies only. Always follow the dates and guidance given by your midwife, obstetrician or doctor, and contact them promptly about bleeding, pain, reduced movements, or anything else that worries you — including things you are not sure are worth mentioning. Learn more about how we build these tools or read our privacy policy.