Pregnancy Calculator
This pregnancy calculator estimates your due date and shows how far along you are. Enter the first day of your last period, or a known conception date, and you'll see the due date, your current week and trimester, and how many days are left. You can adjust your cycle length too. It's a planning estimate, so treat the date as a window, not a promise, and you'll confirm it with your provider.
Last updated June 17, 2026 40 weeks from the last period Reviewed by the Calcowa team
Pick a valid date in the past.
Cycle length applies to the last-period method. Leave the as-of date on today for your current week.
last period + 280 days = Oct 8, 2026
How does a pregnancy calculator work?
Pregnancy is dated from the first day of your last period, and the classic estimate, Naegele's rule, adds 280 days, which is 40 weeks. If you know when you conceived, the same due date is that day plus 266 days, since conception usually falls about two weeks after the period starts. This pregnancy calculator handles both, and it nudges the date when your cycle runs longer or shorter than 28 days. From the start date it also counts forward to today, so you'll see how many weeks and days along you are and which trimester that lands in. It's a planning estimate, not a promise, and a first-trimester ultrasound dates things more precisely, so it's the figure your provider will trust over this one.
Estimating a due date, step by step
Here's the last-period method for a period starting January 1 with a 28-day cycle. It's the same path the tool runs, and you'll see it match the boxes above:
- 1
Start from the last periodUse the first day of your most recent period as day zero.
- 2
Add 280 daysForty weeks later lands the estimated due date, about October 8.
- 3
Adjust for the cycleIf your cycle isn't 28 days, the date shifts by the difference.
- 4
Count to todayDays from the start to today give your weeks along and trimester.
Weeks, trimesters, and milestones
Here's how the weeks group into trimesters across the roughly 40-week span. The boundaries are the common ones, and they're not universal, so don't be surprised if your provider's chart shifts a week here or there.
| Trimester | Weeks | Roughly |
|---|---|---|
| First | 1 to 13 | Months 1 to 3 |
| Second | 14 to 27 | Months 4 to 6 |
| Third | 28 to 40 | Months 7 to 9 |
| Full term | 39 to 40 | Around the due date |
Frequently asked questions
The standard method, Naegele's rule, adds 280 days, or 40 weeks, to the first day of your last period. If you know the conception date instead, it's that date plus 266 days. This pregnancy calculator uses both, and it adjusts for a cycle longer or shorter than 28 days. The result is an estimate, since only about 1 in 20 babies arrive on the exact date.
Pregnancy is counted in weeks from the first day of your last period, not from conception, which is roughly two weeks later. So at 12 weeks pregnant, the baby's been developing for about 10 weeks. The tool shows your weeks and days, the trimester, and how many days are left, all measured to the date you choose, which defaults to today, so you'll always see where you are right now.
The first trimester runs through week 13, the second covers weeks 14 to 27, and the third is week 28 until birth. Each brings different milestones, and the calculator labels which one your current week falls in. They split the roughly 40-week span into three stretches, though they're not identical everywhere, since the exact week boundaries vary a little between sources.
It's a solid estimate, not a guarantee. Around 1 in 20 births land on the predicted day, while most arrive within two weeks either side. A dating ultrasound in the first trimester is more precise than the last-period method, especially when cycles are irregular. Treat the date as a target window, and your provider will confirm and adjust it.
Then the last-period estimate shifts. A longer cycle means you likely ovulated later, so the due date moves back, and a shorter cycle moves it forward. The tool lets you set your cycle length and adjusts the date by the difference from 28 days. If you're unsure of your cycle, the conception-date mode sidesteps it.
No. This pregnancy calculator gives general estimates for planning and is not medical advice, a diagnosis, or a substitute for care. Your doctor or midwife will date the pregnancy with an exam and ultrasound, and their figure is the one to follow, not this one. If you've got any concern about your pregnancy, don't wait, and contact a qualified healthcare provider.
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