How your due date is calculated

Your due date is one of the first things you’ll want to know — it anchors the whole journey. But it helps to understand from the start that it’s an estimate, not an appointment. Only a small proportion of babies actually arrive on the day, so think of it as the middle of a window rather than a deadline.

The 40-week convention. Pregnancy is counted as roughly 40 weeks from the first day of your last menstrual period (LMP) — not from conception. That sounds odd, since you weren’t actually pregnant in those first couple of weeks, but dating from the LMP is the standard because it’s a date most people can identify, whereas the exact moment of conception usually isn’t known.

The quick calculation. A common rule of thumb (Naegele’s rule) is to take the first day of your last period, add one year, subtract three months, and add seven days. In practice your care team, or a pregnancy app, does the maths for you — but that’s where the number comes from.

Why the dating scan matters. If your periods are irregular, or you’re not sure of your LMP, the dates can be off. That’s why an early ultrasound (the dating scan, usually around 8–13 weeks) is the most accurate way to estimate your due date — it measures your baby’s size, which early on is a reliable guide to gestational age. If the scan date differs from your LMP date by more than a few days, your team will usually go with the scan.

IVF is different. If you conceived through IVF, your due date is calculated from the exact date of egg collection or embryo transfer, which makes it more precise than an LMP estimate.

Why it’s only an estimate. Cycles vary, ovulation timing varies, and babies come when they’re ready — most arrive somewhere between 37 and 42 weeks. Going a little “over” is completely normal, especially with a first baby, so try not to fixate on the exact date as it approaches.

What the date is used for. Beyond the excitement, your due date guides the timing of scans, tests and appointments, helps track your baby’s growth, and frames decisions later on (like conversations about induction if you go well past term). It’s a useful anchor for your care, even though your baby hasn’t read the calendar.

It’s also worth knowing your due date can be adjusted during pregnancy — most often after the dating scan, but occasionally later if your baby’s growth suggests it — so don’t be thrown if the number shifts by a few days. So enjoy having a date to focus on — just hold it loosely. Your midwife or GP will confirm and, if needed, adjust it after your dating scan, and from there it quietly shapes your pregnancy care right through to meeting your baby, whenever they decide to arrive.