Every due date is built on the same question: when did the pregnancy actually start? Different methods answer it with different certainty. Understand what each one measures, and you'll know exactly how much to trust the number on the screen.
The four ways to date a pregnancy
Depending on what you know, you can date your pregnancy by your last period, your ovulation day, an IVF transfer or an early ultrasound. Each shifts the starting point, and each has a different margin of error.
| Method | Start point | Confidence |
|---|---|---|
| Last period (LMP) | A remembered date | Good for regular 28-day cycles |
| Ovulation day | When the egg was released | Better, if you tracked it |
| Early ultrasound | Measured embryo size | Very good in the first trimester |
| IVF transfer | Exact transfer date | Best of all |
Last period: the everyday default
Naegele's rule adds 280 days to the first day of your last period. It assumes a 28-day cycle with ovulation on day 14. For many people that's exactly right; for others with longer or irregular cycles, it drifts by several days. It's the standard because it's the date almost everyone can recall.
Ovulation: a sharper starting line
If you tracked ovulation — via temperature, ovulation kits or an app — you have a much better start point. Dating from ovulation adds 266 days (38 weeks). Compared with LMP, this removes the guess about cycle length. It's a solid upgrade for people who know they ovulated on day 17 or day 12 rather than day 14.
Ultrasound: the reliable mirror
An early scan measures the embryo's crown-rump length to estimate age. Because it observes reality rather than assuming a cycle, a first-trimester scan is trusted within a week or even a few days. That's why care teams often revise the LMP-based date after an early scan. Later scans, by contrast, are far better at checking growth than at dating.
IVF: the gold standard
With IVF, nothing is guessed. The transfer date is recorded exactly, and the embryo's age is known precisely — a day-5 blastocyst is counted as 2 weeks 5 days pregnant on transfer day, a day-3 embryo as 2 weeks 3 days. There's no cycle length, no ovulation uncertainty. If any method deserves the word "accurate," this is it.
So which should you trust?
If you had IVF, use the transfer-based date and take your scan readings as confirmation. If you tracked ovulation, prefer the ovulation-based date. If your cycle is regular, LMP works fine — and your early scan will usually fine-tune it. Put your dates into the due date calculator and see how the methods agree.
One honest caveat: most due dates land within a week or two of the real birth day, no matter how accurate the method. Pregnancy reliably makes plans, and then gently ignores them.