How to use it· 5 steps
- Choose how to date the pregnancy: your last period, a conception date, an IVF transfer or an ultrasound scan.
- Enter the date — plus your usual cycle length, the embryo's age at transfer, or the gestational age the scan reported.
- Read your estimated due date, how many weeks and days pregnant you are today, your trimester and the days remaining.
- Scroll the timeline for the dates of the screening windows, the trimester changes and the start of term.
- Have a scan and a period date? Enter both — the calculator applies the guidance clinicians use to choose between them.
Step by step
How to use the due date calculator
- 1Choose how to date the pregnancy: your last period, a conception date, an IVF transfer or an ultrasound scan.
- 2Enter the date — plus your usual cycle length, the embryo's age at transfer, or the gestational age the scan reported.
- 3Read your estimated due date, how many weeks and days pregnant you are today, your trimester and the days remaining.
- 4Scroll the timeline for the dates of the screening windows, the trimester changes and the start of term.
- 5Have a scan and a period date? Enter both — the calculator applies the guidance clinicians use to choose between them.
How a due date is calculated
By convention a pregnancy lasts 280 days — 40 weeks — counted from the first day of the last menstrual period, not from conception. This is Naegele's rule, and it is why 'weeks pregnant' includes about two weeks before conception happened: in a textbook 28-day cycle ovulation falls around two weeks after the period starts, so a baby conceived then is already counted as two weeks along.
The rule is often taught as a shortcut — add seven days, subtract three months, add a year — but months are different lengths, so the shortcut can drift by a day or two. From a period that started on 1 March 2025 it gives 8 December; counting 280 days gives 6 December, which is what pregnancy wheels and clinical software use. This calculator counts days, leap years included.
Cycle length shifts the date. A longer cycle usually means later ovulation, so the calculator moves the due date by the difference from 28 days: a 32-day cycle adds four days, a 25-day cycle takes off three. The correction assumes your cycles are regular; if they are not, the period date is a weak anchor and an early scan is the better one.
The other starting points follow the same logic. From a known conception date, the due date is 266 days (38 weeks) later. After IVF it is set from the transfer and the embryo's age — 261 days after a day-5 blastocyst transfer and 263 days after a day-3 embryo transfer, as the American College of Obstetricians and Gynecologists (ACOG) recommends. From an ultrasound, it is the scan date plus 280 days minus the gestational age the scan measured.
Why an early scan usually wins
Measuring the embryo by ultrasound in the first trimester — up to 13 weeks and 6 days — is the most accurate way to establish or confirm a due date, accurate to within about five to seven days.
When a scan and a period date disagree, ACOG's guidance (Committee Opinion 700) is to keep the period-based date unless the gap exceeds a threshold that widens as pregnancy progresses: more than 5 days before 9 weeks; more than 7 days from 9 weeks to 15 weeks 6 days; more than 10 days from 16 weeks to 21 weeks 6 days; more than 14 days from 22 weeks to 27 weeks 6 days; and more than 21 days from 28 weeks on. The widening reflects how much babies' sizes vary later in pregnancy. If you enter both dates, the calculator applies this rule and tells you which one it kept.
Two consequences follow. Once an accurate early scan has set the date, later scans are generally used to check growth rather than to change it. And a pregnancy without a scan confirming or revising the due date before 22 weeks is, in ACOG's words, suboptimally dated.
Weeks, trimesters and what 'term' means
Gestational age is written as weeks plus days: 13+6 means 13 weeks and 6 days. ACOG divides pregnancy into the first trimester up to 13+6, the second from 14+0 to 27+6, and the third from 28+0 until birth.
Term is a window, not a day. 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 as 42+0 and beyond. Birth before 37+0 is preterm. The due date, 40+0, sits inside full term.
The timeline lists the dates that usually matter: first-trimester screening, offered in many countries between 11+0 and 13+6; the anatomy scan, usually between 18 and 22 weeks; 24 weeks, widely used as a threshold of viability; gestational diabetes screening, commonly at 24–28 weeks; and the term milestones. Schedules vary by country and clinic, and your care team's plan comes first.
On viability, carefully: babies born at 24 weeks have a substantial chance of survival with neonatal intensive care, and outcomes improve with every additional week. ACOG and the Society for Maternal-Fetal Medicine call birth between 20+0 and 25+6 'periviable'; what is possible in that window depends heavily on the exact week, the baby and the hospital.
How exact is a due date?
Not very, and it is not meant to be. Only a few per cent of babies are born on their due date itself; most arrive in the weeks either side. Even when ovulation is pinned down by daily hormone tests, natural pregnancies vary: in a study by Jukic and colleagues (2013), pregnancy lengths ranged over 37 days — more than five weeks — after preterm births were excluded.
So treat the due date as the middle of a window, and plan for the window. Its precision matters clinically: it is how your care team times screening tests, checks a baby's growth and decides when a pregnancy has gone on long enough.
This calculator gives estimates for planning, not medical advice. The due date your midwife or doctor sets — usually confirmed by a scan — is the one to go by, and bleeding, pain or any worry is a reason to contact them, whatever the calendar says.
Answers
Frequently asked questions
How is a due date calculated from my last period?
Add 280 days (40 weeks) to the first day of your last period — Naegele's rule — then shift it by the difference if your cycle is longer or shorter than 28 days. A period that started on 1 January 2026 gives a due date of 8 October 2026 on a 28-day cycle.
How many weeks pregnant am I?
Count from the first day of your last period — or from the adjusted start, if a scan or IVF set your dates. The calculator shows weeks and days, such as 10 weeks, 3 days, as of today.
Why is pregnancy counted from before conception?
Because the first day of a period is a date most people know, while the day of conception rarely is. Counting from the period adds about two weeks before conception, which is why a pregnancy is called 40 weeks long although the baby develops for about 38.
How is an IVF due date worked out?
From the transfer date and the embryo's age: 261 days after a day-5 blastocyst transfer and 263 days after a day-3 transfer, following ACOG. Because the embryo's age is known exactly, IVF dating is the most precise of all.
My scan gave a different due date. Which one is right?
Usually the early scan. ACOG's guidance keeps the period-based date only if the two agree within a threshold — 5 days before 9 weeks, 7 days up to 15+6, then 10, 14 and 21 days later in pregnancy. Enter both and the calculator applies the rule for you.
Is anything I enter saved?
No. The dates are calculated in your browser and are never uploaded or stored.