Pregnancy Due Date Calculator

Babies have their own timeline — anyone who's been through pregnancy will tell you that. But the methods doctors use to estimate when yours will show up are surprisingly reliable. The most widely used approach is called Naegele's Rule, and it's been the standard in obstetrics for well over a century. It works by counting 280 days (40 weeks) from the first day of your last menstrual period. Simple in concept, but remarkably effective.

Of course, not every pregnancy follows the same path. Whether you conceived naturally with regular cycles, tracked your ovulation carefully, or went through IVF, the starting point for the calculation changes — and that's exactly why this calculator gives you three distinct methods.

If your menstrual cycle is unpredictable — and you're not alone in that; roughly 14 to 25 percent of women deal with irregular cycles — the LMP method loses some of its reliability. Here's the thing: Naegele's Rule assumes ovulation happens on day 14, but if you ovulate on day 21 or day 10, that assumption throws the estimate off by a week or more.

A few practical options that actually help:

• Use the Conception Date method if you tracked ovulation with OPK strips, basal body temperature, or cervical mucus changes. This sidesteps the cycle-length problem entirely.
• Average your recent cycles. Look at your last 3 to 6 periods. If you've had cycles of 26, 32, and 29 days, use 29 as your cycle length in the calculator.
• Lean on early ultrasound dating. For women with PCOS, thyroid conditions, or genuinely erratic periods, a scan between 8 and 12 weeks gives the most trustworthy due date. Your provider measures the baby's crown-rump length, and that number doesn't care about your cycle history.

If you went through a frozen embryo transfer (FET), the math is identical to a fresh IVF cycle — the embryo's age at transfer is what matters, not whether it was frozen or fresh. Select the IVF method in our calculator, choose Day 3 or Day 5, and enter the date the embryo was transferred into your uterus.

For intrauterine insemination (IUI), the approach is a bit different. IUI timing is closely linked to ovulation, so the Conception Date method works best. Enter the date of your IUI procedure (which is timed to ovulation), and the calculator adds 266 days. Some doctors prefer to date IUI pregnancies using the LMP method and then confirm with an early scan — either way, you'll get a solid estimate.

Let's set realistic expectations. Studies consistently show that only about 4 to 5 percent of babies arrive on their exact estimated due date. That doesn't mean the calculator is broken — it means human biology has its own schedule.

What the research tells us about accuracy by method:

• LMP method (Naegele's Rule): Accurate within ±5 days for women with regular 28-day cycles. Less reliable if your cycles are irregular or if you're unsure of the exact start date.
• Conception date method: Narrows the window to ±1–2 days, but only if you're confident about when ovulation occurred.
• IVF calculation: The most precise at less than ±1 day, since fertilization timing is known exactly.
• First-trimester ultrasound: Considered the clinical gold standard, accurate to within ±5–7 days — and it's what doctors rely on when period dates are uncertain.

This trips up almost every first-time parent. You conceived 8 weeks ago, but your doctor says you're 10 weeks pregnant. Who's right?

Both, actually. Gestational age is counted from the first day of your last period — it includes roughly 2 weeks before you even conceived. Fetal age (also called embryonic age) counts from fertilization. The medical world uses gestational age because the start of your last period is a date most people remember. Your actual conception date? Much harder to pin down unless you tracked ovulation or did IVF.

So when your pregnancy app says "10 weeks" and your mental math says "8 weeks since conception," you're both correct. All medical charts, lab results, and milestone references use gestational age.

Yes, and it happens more often than you might expect.

• Ultrasound discrepancy: If your first-trimester scan shows the baby measuring significantly ahead of or behind the LMP-based estimate (typically more than 5–7 days off), your provider will revise the due date. This is standard practice, not a cause for concern.
• Irregular cycles: If you ovulate later or earlier than day 14, the LMP method may have started with the wrong assumption. The ultrasound corrects for this.
• Late dating scan: If your first ultrasound doesn't happen until the second trimester, the measurement margin widens — which is why early scans are so valuable for accurate dating.

If you're expecting twins, the estimated due date calculation starts the same way — 40 weeks from LMP or 266 days from conception. But here's the practical reality: twins rarely make it to 40 weeks.

On average, twins arrive around 36 to 38 weeks, and triplets closer to 33 to 34 weeks. Your provider will establish a separate delivery target based on your specific situation — especially whether your twins are identical (monochorionic) or fraternal (dichorionic), since identical twins sharing a placenta carry higher risk factors and are typically delivered earlier.

Our calculator gives you the standard 40-week estimate, which is still useful as a reference point. But for twins, your OB's guidance and regular ultrasound monitoring will give you the actual timeline to plan around. You can also use our pregnancy weight gain calculator to track healthy weight targets for twin pregnancies.

When your dates are uncertain, ultrasound is the most reliable way to establish how far along you are. During a first-trimester scan — ideally between 8 and 12 weeks — the sonographer measures your baby's crown-rump length (CRL), the distance from the top of the head to the bottom.

That single measurement can date a pregnancy with remarkable precision (within about 5 days). After the first trimester, dating accuracy decreases because babies start growing at different rates — a bigger baby at 20 weeks isn't necessarily older, just bigger. Which is exactly why early scans are emphasized for accurate dating.