A pregnancy due date, also called the estimated date of delivery, is one of the first figures an expectant parent receives, and it shapes everything from antenatal appointments to parental leave planning. It is also widely misunderstood: it is an estimate with a range of uncertainty measured in weeks, not an appointment that the baby is expected to keep. Understanding how the date is calculated, what assumptions it rests on, and how it is refined later in pregnancy will help you use it calmly and plan around it sensibly.
The classic calculation is Naegele's rule, named after the German obstetrician Franz Karl Naegele. It assumes a regular twenty-eight-day cycle and estimates the due date as forty weeks, or 280 days, after the first day of the last menstrual period, often abbreviated LMP. The traditional shortcut is: add seven days to the first day of the last period, subtract three months, and add one year. The arithmetic is easy to do on a calendar, which is why the rule survived for two centuries, but the assumptions deserve scrutiny: it assumes ovulation on day fourteen of the cycle, a twenty-eight-day cycle, and an accurate memory of the last period's start date. Where any of those fails, the estimate shifts.
Consider a worked example. Suppose the first day of the last menstrual period is 1 January 2026. Applying Naegele's rule: add seven days to get 8 January, subtract three months to get 8 October, and add one year to get 8 October 2026. Let us verify by counting days directly. Forty weeks is 280 days. Counting forward from 1 January 2026: January has 31 days, so reaching 31 January accounts for 30 days after the start; February 2026 is not a leap year and has 28 days, taking the count to 58; March adds 31, taking it to 89; April adds 30, taking it to 119; May adds 31, taking it to 150; June adds 30, taking it to 180; July adds 31, taking it to 211; August adds 31, taking it to 242; September adds 30, taking it to 272; and 8 more days into October reaches 280, landing on 8 October 2026. Both methods agree, which is a useful cross-check. A second example: if the last period began on 15 March 2026, adding seven days gives 22 March, subtracting three months gives 22 December, and adding one year gives 22 December 2026 as the estimated due date.
Cycle length changes the answer, and this is where many online calculators differ from the simple rule. If cycles are regularly longer than twenty-eight days, ovulation happens later, and the due date should be later by roughly the difference. A person with a regular thirty-five-day cycle ovulates about seven days later than the twenty-eight-day assumption, so the due date estimated from the last period should move about seven days later too. Conversely, a regular twenty-one-day cycle moves the estimate about seven days earlier. Some calculators ask for cycle length and apply this adjustment automatically; others do not, so it is worth checking which kind you are using. Irregular cycles make the last-period method unreliable altogether, and in that case an early ultrasound becomes the better dating tool.
Ultrasound dating is generally more accurate than the last-period method when performed in the first trimester. In early pregnancy, embryos grow at a remarkably consistent rate, so measuring the crown-rump length, the length from head to bottom, allows the gestational age to be estimated within a few days. Later ultrasounds are less precise for dating, because individual variation in growth increases as pregnancy progresses. For that reason, the dating scan in the first trimester is often treated as the reference, and the due date may be revised after it. A revised date is not a mistake in the earlier calculation; it is the estimate being improved with better information, and it is common for the two methods to differ by several days.
Gestational age itself is a convention worth understanding. Pregnancy is counted from the first day of the last period, not from conception, which means the first two weeks of the counted pregnancy occur before conception in the usual twenty-eight-day model. A due date at forty weeks therefore corresponds to about thirty-eight weeks after conception. Trimesters divide those forty weeks into three periods of roughly thirteen weeks each. Babies born between thirty-seven and forty-two weeks are generally described as term, and only a small minority arrive on the exact due date; most arrive within about two weeks either side. That spread is normal biology, not a sign that the calculation was wrong, and it is why experienced advice treats the due date as the middle of a window rather than a deadline.
Practical planning follows from that understanding. Antenatal appointments are scheduled by gestational age, so a revised dating scan can shift the whole schedule slightly. Work leave, childcare arrangements, and travel plans should be built around a window of several weeks rather than a single day, and travel late in pregnancy may be restricted by airlines and insurers, so check those rules well in advance. If labour has not started by the due date, that alone is not usually a cause for alarm: monitoring increases, and options are discussed with the healthcare team, including waiting with surveillance and induction at a point determined by individual circumstances. The due date is the start of a conversation, not a trigger for automatic intervention.
There are situations where professional advice should be sought promptly rather than relying on a calculator. These include bleeding, severe pain, reduced fetal movement later in pregnancy, signs of pre-eclampsia such as severe headache or vision changes, and any concern about the pregnancy's progress. A calculator cannot assess any of these, and it cannot replace antenatal care. If the last period date is uncertain, if cycles are irregular, or if there is any known condition affecting pregnancy, raise it at the first antenatal appointment so that dating can be established properly.
This guide and any pregnancy due date calculator are educational tools providing estimates only. They are not medical advice, they cannot diagnose any condition, and they are not a substitute for professional antenatal care. If you have questions or concerns about a pregnancy, please consult a qualified healthcare professional.
Try it next: use the Pregnancy Due Date Calculator with your own dates, then estimate fertility timing with the Ovulation Calculator and track dates with the Date Calculator. For background reading, see Ideal Weight Explained.