Pregnancy due date calculator
Estimate your due date, current week of pregnancy and trimester from the first day of your last period (LMP).
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How the due date is calculated
This calculator uses Naegele's rule — the standard method used by the NHS and American College of Obstetricians: 280 days (40 weeks) are added to the first day of your last menstrual period, adjusted for your cycle length if it differs from 28 days. The donut above shows your progress through the standard 40-week pregnancy.
Worked example: LMP of March 1
With a last menstrual period of March 1 and a standard 28-day cycle:
- LMP: March 1
- Add 280 days (40 weeks): March 1 + 280 days
- Estimated due date: December 6
If the cycle is longer than 28 days, the calculator adds the extra days on top — a 32-day cycle (4 days longer than average) shifts the same due date to December 10, since ovulation likely happened later in that longer cycle.
The three trimesters
- First trimester (weeks 1–13) — the embryo's major organs and structures form; morning sickness and fatigue are common.
- Second trimester (weeks 14–27) — often called the "easier" stage; energy tends to return and the bump becomes visible.
- Third trimester (weeks 28–40) — rapid growth, more frequent checkups, and preparation for birth.
Curious exactly how many weeks along you are today? See our how many weeks pregnant am I guide.
Key milestones by gestational week
| Week | Trimester | Milestone |
|---|---|---|
| Week 8 | First | Heartbeat typically detectable on ultrasound |
| Week 12 | First | End of first trimester; miscarriage risk drops significantly |
| Week 20 | Second | Anatomy scan; sex often revealed if desired |
| Week 24 | Second | Viability milestone with intensive care support |
| Week 28 | Third | Third trimester begins; checkups become more frequent |
| Week 37 | Third | Considered "early term" — baby's organs are mature |
| Week 40 | Third | Full-term due date |
Why your due date might change
An LMP-based due date is only a starting estimate, and it's normal for that date to shift once more information is available. The most common reason is an early dating ultrasound: measuring the size of the fetus in the first trimester is generally more accurate than counting from the last period, especially if your cycle is irregular or you're unsure of the exact start date. A dating scan can move your estimated due date by anywhere from a few days to one or two weeks in either direction, and once that scan happens, most providers use the scan-based date going forward rather than the original LMP calculation.
Ovulation timing is another factor. Naegele's rule assumes ovulation happens on day 14 of a 28-day cycle, but ovulation can happen earlier or later depending on your natural cycle length, which is why this calculator asks for your average cycle length and adjusts the estimate accordingly.
What happens around your due date
A due date is an estimate, not a deadline. Only about 5% of babies are actually born on their exact due date, and most arrive within a two-week window either side of it, so full-term birth covers a fairly wide range of days. First babies also tend to run slightly later than subsequent ones, on average.
If a pregnancy continues significantly past the due date, providers sometimes discuss induction, since the risk of complications can rise the further a pregnancy goes past term. The exact point at which induction is considered varies by provider and by the specific circumstances of the pregnancy, so this is a conversation to have directly with your midwife or doctor rather than something a calculator can determine.
Frequently asked questions
How is a due date calculated?
LMP date + 280 days (Naegele's rule), adjusted for cycle length. Only ~5% of babies arrive exactly on the due date.
How many weeks pregnant am I?
Weeks are counted from the first day of your last period — the calculator shows your exact week above.
How accurate is this?
Usually within 1–2 weeks. An early ultrasound gives the most accurate dating — always confirm with your midwife or doctor.
What if my cycle isn't 28 days?
Select your average cycle length above — the calculator adjusts the due date accordingly, since ovulation timing shifts with cycle length.
Can I use this if I don't remember my exact LMP date?
Enter your best estimate of the first day of your last period. If you're unsure, an early dating ultrasound from your provider will give a more precise due date based on the size of the fetus, and that scan date typically takes priority over an LMP-based estimate.
This tool is for information only and is not medical advice. Always consult your midwife or doctor.