Finding out roughly when your baby will arrive is one of the first exciting milestones of pregnancy. Your “due date” is really an estimated date of delivery (EDD) — a single best-guess day at the middle of a natural range — not a fixed appointment your baby has to keep. Here are the three methods providers use to work it out, from the most common to the most accurate.
1. How do you calculate a due date from your last period?
The most common method is Naegele’s rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). A quick way to do the same sum in your head:
- Take the first day of your last period.
- Subtract 3 months.
- Add 7 days (and one year).
So if your last period started on 10 January, subtracting 3 months gives 10 October and adding 7 days lands on 17 October as your due date. This shortcut assumes a regular 28-day cycle with ovulation on day 14.
If your cycle is longer or shorter, the estimate shifts — a 35-day cycle, for instance, pushes your due date roughly a week later. Our Due Date Calculator lets you adjust your cycle length so the estimate fits you, not an average.
2. How do you calculate a due date from conception or IVF?
If you know the day you conceived — for example through ovulation tracking — your due date is about 266 days (38 weeks) later, since conception happens roughly two weeks after your period starts. This can be more precise than the LMP method because it doesn’t rely on assumptions about your cycle. (Tracking ovulation next time? Our Ovulation Calculator can help.)
IVF dates are the most precise of all. Because the exact date of egg retrieval or transfer is known, your clinic can give a due date accurate to the day — counting 38 weeks from a day-5 embryo transfer, for example. If you conceived through IVF, use the date your clinic gives you over any LMP estimate.
3. How accurate is a dating ultrasound?
An early ultrasound (ideally between 8 and 14 weeks) measures your baby and is considered the most accurate way to date a pregnancy. In the first trimester all babies grow at a similar rate, so the measurement can pin down your dates to within a few days. If the scan differs significantly from your LMP estimate, your provider will usually re-date the pregnancy from the scan — that’s routine, not a cause for worry.
What if you don’t know your last period date?
It’s very common not to remember the exact day — especially with irregular cycles, or if you conceived soon after stopping the pill. Don’t worry: the dating ultrasound is designed exactly for this. Because early growth is so predictable, a scan can establish your due date even with no reliable LMP at all. If you can only recall roughly when your last period was, give your provider your best estimate and the scan will refine it.
Why is your due date only an estimate?
Only about 1 in 20 babies are born on their exact due date. A normal, full-term birth can happen anytime between 37 and 42 weeks — so think of your due date as the middle of a window, not a deadline.
A full-term birth can happen anywhere along this band.
Here’s how the three methods compare at a glance:
| Method | What it needs | How accurate |
|---|---|---|
| Last period (LMP) | First day of your last period | Good with regular cycles; less so if irregular |
| Conception date | A known ovulation/conception day | More precise — skips cycle guesswork |
| IVF transfer | Egg retrieval or transfer date | Accurate to the day |
| Dating ultrasound | An early scan (8–14 weeks) | The most accurate overall |
Whichever method you start with, an early scan usually has the final say — so don’t be surprised if your date is nudged by a few days at your first appointment.
When will you get your “official” due date?
You can estimate your date at home from day one, but it’s usually confirmed at your first ultrasound, often around 8–14 weeks. That early scan measures your baby and either backs up your LMP estimate or gently adjusts it. From then on, that scan-based date is the one your care team uses for appointments, screening windows and growth checks.
So it’s completely normal to carry a rough date for the first few weeks and have it firmed up later — the early estimate is for you, the scan date is the one in your notes.
What should you do with your due date?
Treat it as a planning anchor, not a countdown to one exact day:
- Plan around the window, not the date. Telling people “early October” rather than a single day saves a lot of “any news yet?” messages near the end.
- Use it to time the practical stuff — when to wind down work, prepare the nursery, and pack your bag.
- Hold it loosely. First babies in particular often arrive a little later, and anything from 37 to 42 weeks is full term.
Follow along week by week with our Pregnancy Week Tracker, which shows your current week and what’s developing. Want a personalised estimate in a few seconds? Try the Due Date Calculator.
This article is for general information only and isn’t medical advice. Your healthcare provider is the best source for questions about your due date and pregnancy.