Pregnancy Due Date Calculator — estimate your baby's arrival

This pregnancy due date calculator estimates when your baby is likely to arrive, using either the first day of your last menstrual period or a known conception or IVF transfer date. It also shows your current gestational week, trimester, estimated conception date, and how many weeks are left — all calculated instantly from simple, transparent date math.

Arb Digital built this due date calculator as one of a handful of free pregnancy and cycle tools, so anyone can get a quick, reliable estimate without downloading an app or handing over an email address.

What This Due Date Calculator Does

Enter either the first day of your last period plus your average cycle length, or a known conception/IVF transfer date, and the calculator projects your estimated due date along with a full snapshot of where you are right now: current week and day, which trimester you're in, roughly when conception happened, and how many weeks remain until your due date. Switching between the two calculation modes updates every field instantly, so you can compare both methods if you happen to know both dates.

The calculator is built around the standard clinical convention used by obstetric providers in the United States, so the numbers it gives you should closely match what you'd hear at a prenatal appointment, at least until an ultrasound provides a more precise dating estimate.

How to Use the Due Date Calculator

1. Choose your calculation method. Pick "Last menstrual period" if you know the first day of your last period, or "Conception or IVF transfer date" if you know (or were told) a more precise conception date.
2. Enter the date. For LMP mode, also enter your average cycle length if it isn't close to 28 days — this shifts the estimate to better match your body's actual timing.
3. Click "Calculate Due Date." You'll instantly see your estimated due date, current week and trimester, estimated conception date, and weeks remaining.
4. Compare with your ultrasound dating, if you've had one. A first-trimester ultrasound is generally considered more accurate than LMP-based math alone, so if your provider has already given you a due date from a scan, that number should take priority over this estimate.

The Formula — Naegele's Rule

The default method used here, and by most clinicians, is known as Naegele's rule: take the first day of the last menstrual period and add 280 days (40 weeks). If your cycle length differs from the standard 28 days, the calculator adjusts by adding or subtracting the difference, since a longer cycle generally means ovulation — and therefore conception — happened later than it would in a 28-day cycle. When you calculate from a known conception or IVF transfer date instead, the calculator works forward 266 days from conception (38 weeks), or adjusts appropriately for a known-age embryo transfer, since conception itself already accounts for the variable first half of the cycle. This approach mirrors guidance from the American College of Obstetricians and Gynecologists (ACOG).

Why So Few Babies Arrive On Their Exact Due Date

It surprises a lot of expecting parents to learn that only around 4% of babies are actually born on their calculated due date. That's not a flaw in the math — it's simply how the estimate is designed to work. A due date is the mathematical center of a normal-length pregnancy, not a prediction of a single day. Full-term birth is generally defined as anywhere from 37 to 42 weeks of pregnancy, a five-week window during which the vast majority of babies are born completely healthy. Because ovulation timing, individual gestation length, and even labor triggers vary naturally from person to person, due date calculators — including this one — are best treated as a useful planning anchor rather than an appointment on the calendar.

Gestational Age vs. Fetal Age — A Common Point of Confusion

One detail that trips up nearly everyone the first time they encounter it: "gestational age," the number your provider uses and the one this calculator reports, is counted from the first day of your last period — which is roughly two weeks before conception actually happened. "Fetal age" (or conceptional age), by contrast, is counted from the actual moment of conception. That means at "6 weeks pregnant" by gestational age, the embryo itself is closer to 4 weeks old developmentally. Clinicians almost universally use gestational age because the exact moment of conception usually isn't known with certainty, while the first day of the last period usually is. This calculator follows that same clinical convention, and shows your estimated conception date separately so the distinction is clear rather than hidden inside a single number.

Why an Ultrasound Can Beat This Calculator

Date-based due date estimates depend heavily on accurate recall of your last period and a fairly regular, well-understood cycle — two things that aren't true for everyone. A dating ultrasound performed in the first trimester, ideally before 14 weeks, measures the embryo or fetus directly and is considered the most accurate method of establishing gestational age and due date, particularly for anyone with irregular cycles, uncertain last-period dates, or a history of irregular ovulation. If an early ultrasound gave you a due date that differs from what this calculator shows, the ultrasound-based date is generally the one your provider will use going forward.

What Happens Each Trimester

The 40-week pregnancy timeline is traditionally split into three trimesters, and this calculator's "trimester" result maps directly onto that convention. The first trimester runs from week 1 through week 12, and it's the period when the embryo implants, major organs begin forming, and the risk of early miscarriage is highest — many providers hold off on public announcements until this window has passed. The second trimester spans weeks 13 through 27 and is often described as the most comfortable stretch, when early pregnancy symptoms like nausea typically ease, fetal movement becomes noticeable, and an anatomy ultrasound around 18–22 weeks checks growth and development in detail. The third trimester runs from week 28 to birth, generally around week 40, and is marked by rapid weight gain for the baby, more frequent prenatal visits, and preparation for labor. Knowing which trimester you're in — not just which week — gives useful context for what your body and your baby are likely doing right now.

Why Cycle Length Changes the Math

Naegele's rule assumes a textbook 28-day cycle with ovulation occurring on day 14. In reality, cycle length varies widely between individuals, and even from month to month for the same person, which is why this calculator lets you adjust it. If your average cycle runs 32 days instead of 28, ovulation — and therefore conception — most likely happened about four days later than the textbook assumption, so your due date shifts about four days later too. The opposite is true for shorter cycles: a 24-day average cycle generally means earlier ovulation, pulling the estimated due date about four days earlier. This adjustment is a meaningful refinement over a plain LMP-plus-280-days calculation, especially for anyone whose cycles reliably run longer or shorter than average, though it still can't account for the natural cycle-to-cycle variation almost everyone experiences.

Common Mistakes to Avoid

• Entering the wrong date for LMP mode. It should be the first day your last period started, not when it ended or when you took a pregnancy test.
• Forgetting to adjust cycle length. If your cycles regularly run longer or shorter than 28 days, leaving the default in place will skew the estimate by several days.
• Treating the due date as a deadline. Labor beginning a week or two before or after the estimated date is completely normal and expected.
• Ignoring a confirmed ultrasound date. If a dating scan gave you a different due date, that measurement generally overrides date-based math.
• Mixing up gestational age and fetal age. "8 weeks pregnant" does not mean the embryo is 8 weeks old — it's roughly 6 weeks from conception.

Related Free Tools From Arb Digital

Once you have an estimated due date, try the pregnancy week calculator to see your progress in detail, the conception date calculator for a closer look at when pregnancy began, or the ovulation calculator and period calculator if you're still tracking your cycle. Browse every free calculator we've built in our free online tools hub.

Frequently Asked Questions

This tool provides general estimates for educational purposes only and is not medical advice. Dates are approximate — every pregnancy and cycle is different, so confirm with your doctor or midwife.