Due Date Calculator

How this due date calculator works

There are four ways in. Most people use the first day of their last menstrual period (LMP), because that is the date doctors and midwives start from too. If you know when you conceived — for example from ovulation tests — you can use that instead. If you conceived through IVF, the transfer date and the age of the embryo (day 3 or day 5) give the most precise estimate of all. And if you already have a due date from an ultrasound, choose "I know my due date" and we simply lay out your weeks and milestones from it.

The result shows your estimated due date, how many weeks and days pregnant you are today, which trimester you're in, and a dated timeline: when a cell-free DNA blood test becomes possible, the end of the first trimester, the anatomy scan window, the 24-week viability milestone, the start of the third trimester and the full-term window. Everything is calculated on your own device. Nothing you enter is sent to us or stored anywhere.

Naegele's rule, and what happens if your cycle isn't 28 days

The standard method is called Naegele's rule: take the first day of your last period and add 280 days — 40 weeks. That is the convention used by the American College of Obstetricians and Gynecologists (ACOG) and by the NHS. The rule quietly assumes a 28-day cycle, with ovulation (and so conception) around day 14.

Many people don't have 28-day cycles. If yours is regularly longer, you probably ovulate later, so your baby is likely to be due later too; if it's shorter, earlier. This calculator moves the due date by the difference: a 32-day cycle adds 4 days, a 25-day cycle takes 3 away. This adjustment only makes sense for cycles that are fairly regular. If your cycles vary a lot, or you conceived soon after stopping hormonal contraception or while breastfeeding, the LMP method is less reliable and an early ultrasound will date the pregnancy better.

From a conception date we add 266 days (38 weeks). For IVF, the embryo's age is known exactly: a day-5 blastocyst transfer means the due date is the transfer date plus 261 days, and a day-3 transfer plus 263 days.

Why pregnancy is counted from your period, not from conception

It surprises almost everyone: by the convention every clinic uses, you are already "two weeks pregnant" on the day you conceive. Pregnancy weeks — the gestational age — are counted from the first day of the last period, simply because that date is usually known while the day of conception usually isn't. So when this calculator says you are 8 weeks and 3 days pregnant, the embryo itself is about 6 weeks old.

That is also why the numbers on pregnancy apps, scan reports and this page all line up: they share the same starting point. If you entered a conception or IVF date, we convert it to that same scale before showing weeks and milestones.

How accurate is a due date?

A due date is an estimate, not an appointment. Only around 4–5% of babies are born on their due date. Most arrive in the weeks either side of it, and a pregnancy is considered full term from 39 weeks 0 days to 40 weeks 6 days (ACOG). Births from 37 weeks are early term, and after 41 weeks late term.

Your first ultrasound often refines the date. ACOG advises that an ultrasound in the first trimester is the most accurate way to establish or confirm the gestational age, and that when the scan and the LMP date disagree by more than a set margin (about a week for a scan between 9 and 14 weeks), the due date should be changed to match the scan. In the UK the NHS dating scan between 10 and 14 weeks does the same job. Once your clinician has set your due date, use theirs — and pick "I know my due date" above to see your timeline from it.

This calculator is for information only. It cannot tell you anything about your health or your baby's, and it is not a substitute for care from your doctor or midwife.

Your pregnancy milestones, week by week

From 10 weeks: a cell-free DNA blood test (often called NIPT) can be done. It is mainly a screening test for some chromosomal conditions, but it can also report the baby's sex chromosomes if you ask. Whether it's offered, and whether it's covered, depends on where you live.

13 weeks 6 days: the last day of the first trimester. The second trimester runs from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks until birth.

18 to 22 weeks: the anatomy scan (also called the anomaly or mid-pregnancy scan; the NHS offers it between 18 weeks and 20 weeks 6 days). It checks how the baby is growing and developing — and it is when many parents find out the sex, if they want to and if the baby cooperates. Some hospitals don't tell the sex as policy, so ask ahead if it matters to you.

24 weeks: widely used as the threshold of viability — the point from which babies born very early have a real chance of survival with intensive neonatal care. Every extra week in the womb still makes a big difference.

39 weeks 0 days to 40 weeks 6 days: full term. Your due date sits at 40 weeks 0 days.

Planning a gender reveal around your scan dates

Your timeline tells you when you could know: from about 10 weeks with a blood test, or at the anatomy scan around 18–22 weeks. Many parents ask the sonographer to write the result on a card and seal it in an envelope, then hand it to a friend who plans the reveal. Pencil the party in for a week or two after your scan date — enough time for everyone to take their guesses, not so long that the secret leaks.

If family lives far away, you don't need a venue. Diaper Reveal is a free online gender reveal party: everyone joins from their own phone on live video, votes team boy or team girl, plays a short race and a couple quiz, and sees the reveal at the same moment — with a highlight video at the end. You can set it up in a couple of minutes, even before you know the result.

Due date FAQ