Pregnancy Due Date Calculator
An estimate to plan around — your dating ultrasound is the date that counts.
Work out your estimated due date (EDD) from the first day of your last period using Naegele's rule, or from a known conception date or IVF transfer — and see how many weeks pregnant you are today and which trimester you're in.
A due date is an estimate from your dates — a planning guide only. A dating ultrasound is definitive; always follow your doctor, OB-GYN or midwife.
How a due date is worked out
Naegele's rule
The standard obstetric method is Naegele's rule: your estimated due date (EDD) = the first day of your last menstrual period (LMP) + 280 days, which is 40 weeks. The classic shortcut does the same sum by hand — take the first day of your last period, subtract three months and add seven days. It assumes a regular 28-day cycle with ovulation around day 14, so the estimate is only as reliable as those assumptions.
Worked example
If the first day of your last period was January 1, subtract three months (October 1) and add seven days to get a due date of about October 8 — the same result as counting 280 days forward.
This is an estimate, not a diagnosis. Only about 1 baby in 20 arrives on the exact date, and a full-term birth is anywhere from 37 to 42 weeks. A dating ultrasound is definitive and its date takes over from the period-based estimate — always follow the due date your doctor, OB-GYN or midwife gives you.
Cycle length, a known conception date and IVF
Naegele's rule assumes a 28-day cycle, but real cycles vary — and if you know exactly when conception happened, you can date the pregnancy more precisely.
- Cycle length — a longer cycle means later ovulation, so the due date moves back. A 35-day cycle pushes the estimate roughly 7 days later than the standard 28-day assumption; a short cycle brings it forward.
- Known conception date — +266 days (38 weeks). Conception normally happens about two weeks after the first day of your last period, which is why the conception estimate is about two weeks shorter than the 280-day LMP method but lands on a similar date.
- IVF transfer — the embryo's age is known exactly, so the estimate is tighter. Add 261 days for a Day-5 blastocyst transfer, or 263 days for a Day-3 embryo transfer.
Switch the "Calculate from" option to match what you know, then set your average cycle length if you're using your last period and it isn't 28 days.
How many weeks pregnant am I, and the trimesters
Gestational age is counted from your last period
Pregnancy is dated from the first day of your last period, not from conception. Because conception happens about two weeks later, you're already considered about two weeks "pregnant" at the moment of conception — a quirk of the convention that surprises many people. This calculator shows how many weeks and days along you are today.
The three trimesters
Pregnancy is divided into three stages, counted in gestational weeks:
- First trimester — weeks 1 to 13.
- Second trimester — weeks 14 to 27.
- Third trimester — weeks 28 to 40 (and up to 42 at full term).
Remember the date is only an estimate — a normal, full-term birth is anywhere from 37 to 42 weeks, and only about 1 baby in 20 arrives on the exact due date.
❓ Frequently asked Frequently asked questions
How is a due date calculated?
The standard method is Naegele's rule: your estimated due date is the first day of your last menstrual period (LMP) plus 280 days, which is 40 weeks. The classic shortcut is to take the first day of your last period, subtract three months and add seven days. It assumes a regular 28-day cycle with ovulation around day 14, so if your cycle is longer or shorter the estimate shifts — a 35-day cycle pushes the due date about a week later. For example, an LMP of January 1 gives a due date of about October 8.
How accurate is a due date?
A due date is only an estimate — only about 1 baby in 20 actually arrives on the exact date. A full-term, normal birth is anywhere from 37 to 42 weeks. The LMP method also relies on you remembering the date and having a regular cycle. That's why an early dating ultrasound, usually done in the first trimester, is used to confirm or revise the due date; the ultrasound date takes over when it differs from the period-based estimate. Always follow the date your doctor, OB-GYN or midwife gives you.
How do I calculate a due date from conception?
If you know the date of conception — for example from tracking ovulation — add 266 days (38 weeks). Conception normally happens about two weeks after the first day of your last period, which is why the conception estimate is roughly two weeks shorter than the 280-day LMP method. Both should land on a similar due date if your cycle is close to 28 days.
How do IVF due dates work?
IVF gives a very precise due date because the embryo's age is known exactly. For a Day-5 blastocyst transfer, add 261 days to the transfer date; for a Day-3 embryo transfer, add 263 days. Because the timing isn't estimated from a period, IVF due dates are usually more accurate than the LMP method.
How many weeks pregnant am I?
Gestational age is counted from the first day of your last period, not from conception — so at the moment of conception you're already considered about two weeks pregnant. Pregnancy is divided into three trimesters: the first is weeks 1 to 13, the second is weeks 14 to 27, and the third is weeks 28 to 40. This calculator shows how many weeks and days along you are today and which trimester that falls in.
Where these figures come from
The method here is the mainstream obstetric standard for dating a pregnancy. It is an estimate — a dating ultrasound remains the definitive measure.
- Naegele's rule — estimated due date = first day of the last menstrual period + 280 days (40 weeks), assuming a regular 28-day cycle.
- Cycle-length adjustment — the estimate shifts by the difference between your cycle and 28 days (a 35-day cycle adds about 7 days).
- Conception dating — conception date + 266 days (38 weeks).
- IVF dating — transfer date + 261 days for a Day-5 blastocyst, or + 263 days for a Day-3 embryo.
- Trimesters — first weeks 1–13, second weeks 14–27, third weeks 28–40; full term is 37–42 weeks.
Last checked: July 2026. This is a planning estimate, not medical advice. A dating ultrasound is definitive, and the date your doctor, OB-GYN or midwife gives you always takes precedence — see them for your prenatal care.
Select the question that matches where you are right now.
The headline is your estimated due date (EDD) — the day you'd reach 40 weeks. The breakdown shows how many weeks and days pregnant you are today and which trimester that falls in.
Use it to plan appointments, time off and preparations. Book your first prenatal visit early — your dating ultrasound will confirm or fine-tune the date.
It's not a guarantee or a diagnosis. Babies arrive across a normal 37–42 week window, and only about 1 in 20 come on the exact date. An ultrasound is the definitive measure.
Gestational age is counted from the first day of your last period, so you're "two weeks pregnant" at conception. It keeps everyone measuring from the same, more reliably known date.
A few things move the estimate: which date you start from, your cycle length, and whether it's an IVF pregnancy.
Naegele's rule assumes 28 days. A longer cycle means later ovulation, so a 35-day cycle pushes the due date about a week later; a shorter cycle brings it forward.
Last period adds 280 days; a known conception date adds 266; IVF adds 261 (Day-5) or 263 (Day-3). IVF and ultrasound dates are the most precise.
An early ultrasound measures the baby directly and becomes the official due date when it differs from your period-based estimate — usually within a few days for a regular cycle.
A few habits keep the estimate meaningful.
Enter the first day of your last period, not the day it ended. If you know your conception or IVF transfer date, use that option instead for a tighter estimate.
If your cycle isn't 28 days, enter your average — it can shift the due date by several days and matters most for longer or irregular cycles.
Book an early prenatal appointment and a dating ultrasound. The ultrasound date is definitive, so treat this tool as a first estimate to plan around.
Once you have a due date, start planning the arrival — the costs, the childcare, and your own health along the way.
Estimate the cost of a new arrival, from the nursery to the first year.
Baby cost calculator →Childcare is often the biggest line item after the first year — price it early.
Childcare cost calculator →