Your estimated due date is the first day of your last period plus 280 days (40 weeks), adjusted for your usual cycle length. Enter the date and your cycle; the calculator shows the due date, how far along you are today, your trimester, and the key dates of the pregnancy.
How the weeks are counted
Doctors and midwives count pregnancy from the first day of the last menstrual period (LMP), because it is a date most people know. Conception usually happens about two weeks later, around ovulation, so at “6 weeks pregnant” the embryo is about 4 weeks old. That is why a full-term pregnancy is called 40 weeks, although the baby grows for about 38.
Cycle length changes the date
Naegele’s rule assumes a 28-day cycle with ovulation on day 14. With a longer cycle, ovulation comes later, and so does the due date:
| Usual cycle | Due date shift |
|---|---|
| 24 days | 4 days earlier |
| 26 days | 2 days earlier |
| 28 days | – |
| 30 days | 2 days later |
| 32 days | 4 days later |
| 35 days | 7 days later |
Milestones
| Week | What it is |
|---|---|
| End of week 13 | End of the first trimester |
| Week 20 | Halfway; often the anatomy ultrasound |
| End of week 27 | End of the second trimester |
| Weeks 37–38 | Early term |
| Weeks 39–40 | Full term |
| Week 41 | Late term |
| Week 42 and later | Post-term |
The calculator gives the dates of the end of the first trimester and of 39 weeks, so you can plan leave, appointments and family visits.
Only an estimate
A due date is a point in a window, not a deadline. Most babies are born within two weeks either side of it, and first babies tend to come a little later than later ones. When an early ultrasound measures the baby and its estimate differs from the LMP date by more than a few days, your doctor or midwife will usually move the due date to match it.
IVF and known conception dates
If you know the date of conception, the due date is that date plus 266 days. After an embryo transfer, clinics count from the transfer date and the age of the embryo; ask your clinic for the date they use.
Your dates stay on your device: nothing you enter is sent anywhere. Once zet4u is saved there, the calculator works offline too – in a waiting room with no reception, for example.
Frequently asked questions
How is the due date calculated?
By Naegele's rule: the first day of your last menstrual period (LMP) plus 280 days, or 40 weeks. For cycles longer or shorter than 28 days, the date moves by the difference: a 30-day cycle adds 2 days.
How many weeks pregnant am I?
Pregnancy weeks are counted from the first day of your last period, not from conception, which is usually about two weeks later. The calculator shows the weeks and days for today.
How accurate is a due date?
It is an estimate. Only a small minority of babies – often quoted as about 4% – arrive on the due date itself; most come within two weeks of it. An early ultrasound gives the most reliable date.
When does each trimester start?
The first trimester runs to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth.
Sources
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