Due date calculator: Naegele's rule, adjusted for your cycle
Estimate your due date from the first day of your last period and your usual cycle length, with trimester dates and an honest look at accuracy.
Runs entirely in your browser. Nothing you enter is sent anywhere.
The first day of proper bleeding, not spotting.
From the first day of one period to the first day of the next. If you're not sure, leave it at 28.
This tool gives general wellness information, not medical advice. If something feels wrong, talk to your doctor or midwife.
Quick take: Your estimated due date is 280 days after the first day of your last period, which is Naegele's rule. This calculator moves the date by however many days your usual cycle differs from 28. Treat it as a guide: in one large study only about 5% of births happened on the due date.
How it works
A due date gives a pregnancy its shape: the weeks, the trimesters, the scans and checks along the way. It's also only ever an estimate. Here is how this calculator arrives at the date, and how much weight to give it.
How is a due date calculated?
By convention, the estimated due date is 280 days, or 40 weeks, after the first day of the last period. The rule assumes a regular 28-day cycle with ovulation on day 14, so on its own it doesn't allow for cycles of other lengths.[1] Adjusting for your usual cycle length is a simple way to bring the estimate closer to your own rhythm:
Due date = first day of last period + 280 days + (your cycle length − 28 days)
The same adjusted count sets your week of pregnancy today and the dates each trimester begins, so the numbers always agree with each other. Here is how the cycle length changes the result for a last period that started on 1 May 2026:
| Usual cycle length | Adjustment | Estimated due date |
|---|---|---|
| 21 days | 7 days earlier | 29 January 2027 |
| 28 days | None | 5 February 2027 |
| 35 days | 7 days later | 12 February 2027 |
| 42 days | 14 days later | 19 February 2027 |
The calculator accepts cycles from 21 to 45 days. If your cycles are irregular, or you're not sure when your last period started, a dating scan will be far more useful than any formula.
How accurate is a due date?
The Australian study followed 18,708 women whose labour began spontaneously. Only 5% gave birth on the estimated date, whichever dating method was used, and about 66% gave birth within 7 days of it.[2]
Part of this is simply how bodies work. In a study that pinpointed the day of ovulation with hormone tests, the median time from ovulation to birth was 268 days, and the length of pregnancies still spanned 37 days after preterm births were excluded.[3]
The science: What the timing study measured
The Early Pregnancy Study followed 125 naturally conceived pregnancies from conception to birth, identifying ovulation and implantation with urinary hormone measurements. Pregnancy length varied more when it was measured from the last period than from ovulation, reflecting how much the timing of ovulation varies.[3]
Is an ultrasound scan more accurate than a period date?
Period dates depend on memory and on a textbook cycle. Studies summarised in obstetric guidance found that only about half of women accurately recall the first day of their last period, and in one study 40% of women randomly assigned to a first-trimester scan had their due date adjusted because the two dates differed by more than 5 days.[1]
Once you know your due date, the pregnancy week calculator shows where you are today, with the movement guidance for that stage. For checking how hard to exercise, see the pregnancy exercise intensity guide.
Frequently asked questions
Will my due date change during pregnancy?
Usually not once it's been set. Guidance for obstetric care says the due date should be fixed as soon as the period date, the first accurate scan, or both are known, and changed later only in rare circumstances. If your midwife or doctor gives you a date after a scan, that's the one to use.
My pregnancy started with IVF. Should I use this calculator?
It's better not to. After IVF or other assisted reproduction, the due date is worked out from the age of the embryo and the date of transfer, which is more precise than a period date. Your fertility clinic or midwife will give you that date.
Can this calculator tell me when I conceived?
No. Eggsports doesn't estimate ovulation or conception dates, and this tool only works out a due date. If you'd like to know more about your dates, a dating scan and a conversation with your midwife or doctor are the best guides.
Is anything I enter saved or sent?
No. The calculation happens in your browser, and your dates are never stored, sent to us, or added to the page address. Closing the page clears them.
Sources
- American College of Obstetricians and Gynecologists, American Institute of Ultrasound in Medicine, Society for Maternal-Fetal Medicine (2017). Methods for Estimating the Due Date. Committee Opinion No. 700. Obstetrics & Gynecology. doi:10.1097/AOG.0000000000002046
- Khambalia AZ, Roberts CL, Nguyen M, Algert CS, Nicholl MC, Morris J (2013). Predicting date of birth and examining the best time to date a pregnancy. International Journal of Gynecology & Obstetrics. doi:10.1016/j.ijgo.2013.05.007
- Jukic AM, Baird DD, Weinberg CR, McConnaughey DR, Wilcox AJ (2013). Length of human pregnancy and contributors to its natural variation. Human Reproduction. doi:10.1093/humrep/det297