Due Date Calculator
Last updated: 2026-06-25
Your due date is your last menstrual period (LMP) plus 280 days (40 weeks) — Naegele's rule.
Gestational age is counted from the LMP as 0 weeks 0 days, and this calculator also shows your current gestational age and trimester.
Estimate your due date from your last period
Estimated due date
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- Current gestational age
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- Trimester
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- Until due date
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Please enter a valid last period date.
This is a reference estimate based on your last period. Your actual due date and gestational age are adjusted by ultrasound, so confirm with your OB-GYN.
How to use it
- Enter last period date — select the date your last period before pregnancy started.
- Confirm reference date — confirm the date used to count gestational age (defaults to today).
- View result — press Calculate to see your due date, current gestational age and trimester.
How due date and gestational age are calculated
Pregnancy is dated as about 280 days (40 weeks) from the last menstrual period (LMP). Conception actually happens around ovulation, roughly 2 weeks after the LMP, but because the exact date of conception is hard to know, gestational age is counted clinically from the LMP.
Due date = last period start + 280 days (40 weeks)
Current gestational age = (reference date − LMP) ÷ 7 → ○ weeks ○ days
Naegele's rule: LMP + 1 year − 3 months + 7 days
Trimesters by gestational age
| Trimester | Gestational age | Description |
|---|---|---|
| First (early) | weeks 0–13 | Major organ formation |
| Second (mid) | weeks 14–27 | Movement begins, stable phase |
| Third (late) | week 28 – birth | Weight gain, preparing for birth |
| Full term | weeks 37–42 | Most births happen in this window |
Good to know
- Only about 5% of births fall exactly on the due date; most occur within two weeks either side.
- If your cycle isn't 28 days or is irregular, an ultrasound is used to adjust the due date.
- This calculator adds 280 days based on UTC midnight, so results are identical regardless of time zone.
Before pregnancy, use the Ovulation Calculator for your fertile window; to count down to the due date, use the Date Calculator.
Frequently asked questions (FAQ)
How is the due date calculated?
The due date is your last menstrual period (LMP) plus 280 days (40 weeks). This is Naegele's rule, equivalent to taking the LMP, adding one year, subtracting three months, and adding seven days.
How is gestational age counted?
Gestational age is counted from the LMP (set to 0 weeks 0 days), not from conception. It therefore runs about 2 weeks ahead of conception, and the due date is week 40. Gestational age = (today − LMP) ÷ 7.
How are the trimesters divided?
The first trimester is weeks 0–13, the second is weeks 14–27, and the third is week 28 to birth. This calculator also shows which trimester your current gestational age falls in.
Will I deliver on the exact due date?
Only about 5% of births happen exactly on the due date; most occur within two weeks before or after (37–42 weeks is considered full term). If your cycle is irregular or the LMP is uncertain, an ultrasound is used to adjust the date.
Does the time zone change the result?
No. The calculator converts your date to UTC midnight before adding 280 days, so it always shows the same due date regardless of your time zone or daylight saving time.
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Last updated: 2026-06-25