Due Date Calculator

The due date, formally the estimated date of confinement, is an estimate of when a pregnant woman will deliver. It is usually quoted as a single day, though a range is more honest, because only about 4% of births happen on the date itself.

Four methods are in common use, and this calculator supports all of them: last menstrual period, ultrasound, conception date and IVF transfer date.

Last menstrual period

The default assumes childbirth happens on average at a gestational age of 280 days, or 40 weeks, counted from the first day of the last menstrual period.

Due date = last menstrual period + 280 days

There is some debate about when pregnancy technically begins, at fertilisation of the egg or at implantation in the uterus, but gestational age sidesteps it entirely by counting from the last period instead. In gestational terms a pregnancy typically runs 37 to 42 weeks, with 40 used as the estimate.

The method is Naegele's rule, and it carries an assumption worth knowing about: a 28-day cycle with ovulation on day 14. Cycles vary. On a 32-day cycle ovulation lands nearer day 18, which pushes the due date four days later, so this calculator adjusts for the cycle length you enter. On an irregular cycle the whole method weakens, which is one of the reasons a dating scan exists.

Ultrasound

Ultrasound dating uses soundwaves to look inside the body and compares the growth of the fetus against typical growth rates. It is quick, has no known risk to the baby, and gives an accurate estimate early in pregnancy.

Timing is what decides accuracy. A first-trimester scan measures crown-rump length and dates the pregnancy to within about five to seven days. By the third trimester the margin widens to two or three weeks, because babies of the same age differ far more in size the further along they get. This is why an early scan sets the dates, and later scans measure growth against those dates rather than resetting them.

Where a scan and a remembered period date disagree by more than about a week in the first trimester, clinical guidance is to go with the scan.

Conception date

Dating from conception works the same way as dating from the last period, with a difference of about two weeks between the two, which is the gap between the start of the period and ovulation.

Due date = conception date + 266 days

Most people do not know the date of conception. Sperm survive up to five days in the reproductive tract, so intercourse on a known date does not pin fertilisation to that date. Ovulation tracking, whether by temperature, cervical mucus or LH testing, narrows it but rarely fixes it exactly.

IVF transfer date

IVF gives the most precise estimate of the four, because the transfer date is known rather than inferred. The calculation still works to the same average of 40 weeks from a notional last period.

Due date = transfer date + 280 days - 14 days - embryo age

A day-5 blastocyst transferred on 15 June gives a due date of 3 March. The embryo is already five days past fertilisation at transfer, so the notional last period sits 19 days earlier. Day-3 and day-6 transfers shift the date by the corresponding amount, which is why the embryo age matters and the calculator asks for it.

An IVF due date can also be worked out from the day of ovulation, egg retrieval or insemination. The transfer date is used here because it is the one every patient is given.

The due date as a reference point

Where inside the 37 to 42-week window a baby arrives is generally not a cause for concern. The bands matter mostly as a reference for clinicians deciding whether anything needs doing.

Label	Gestational age
Preterm, or premature	before 37 weeks
Early term	37 to 39 weeks
Full term	39 to 41 weeks
Late term	41 to 42 weeks
Post term	after 42 weeks

Babies born anywhere in those ranges can be perfectly healthy, though full-term babies generally have better outcomes than early-term ones, which is why elective delivery before 39 weeks without a medical reason is discouraged.

The bands drive decisions in both directions. If labour starts at 33 weeks, doctors may try to stop it, since a preterm baby faces a set of problems that come from being underdeveloped. If labour has not started by 42 weeks, they may induce. One risk of continuing past 42 weeks is that the placenta, which supplies nutrition and oxygen, can stop working properly while the baby keeps growing and needing more of both, eventually reaching a point where the pregnancy can no longer support it.

How much confidence the date deserves

Fewer than 4% of births land on the estimated date. Roughly 60% happen within a week of it and about 90% within two weeks. Treat the date as the centre of a fortnight-wide window rather than an appointment.

A 2013 study by Jukic and colleagues, which tracked ovulation directly rather than relying on remembered dates, found natural variation in pregnancy length spanning 37 days once preterm births were removed. Over five weeks of genuine biological spread sits behind any single predicted day.

Several things shift the average. First pregnancies tend to run slightly longer than later ones. Older mothers and mothers who were themselves heavier at birth tend towards longer pregnancies. Twins and other multiples arrive earlier, often considerably, and none of the dating here applies to them.

Gestational age against fetal age

Because the count starts at the last period, you are recorded as two weeks pregnant at the moment of conception. Gestational age therefore runs about two weeks ahead of the fetus's real age, and weeks 1 and 2 on any pregnancy schedule happen before there is an embryo at all.

This matters when reading anything clinical. A scan reporting nine weeks means nine weeks of gestational age and an embryo of about seven weeks. Every screening window, every milestone and every date in the schedule below uses gestational age.

What the schedule shows

The table lists all 42 weeks with their date ranges, the trimester each falls in, and the milestones that typically occur. Week 1 begins the day after the last period and week 40 ends on the due date, so the numbering matches how clinicians count.

The size figures are population averages. Healthy babies vary widely around them in both directions without anything being wrong, and the step at week 21 is not a growth spurt but the point where standard tables switch from measuring crown-rump length to crown-heel length.

None of this replaces antenatal care. A due date set by an early scan overrides one calculated from a remembered period date, and the dating your clinician records is the one that counts for every decision that follows.

When a scan overrules the date you counted

Counting from the last period assumes ovulation on day 14 of a 28-day cycle, and real cycles vary. First-trimester ultrasound is more accurate, and gestational age from crown-rump length is good to about 5 to 7 days up to 13 6/7 weeks. ACOG sets out when the scan should replace the counted date, and the threshold widens as pregnancy advances because the measurements lose precision:

Once a due date is set this way it is not usually revised again. A later scan showing a smaller baby is read as a question about growth rather than about the calendar, which is why the dating scan carries so much weight in the schedule that follows.

Term is a range, not a day

ACOG replaced the single label "term" with four, because outcomes differ across those five weeks. Early term runs 37 0/7 to 38 6/7 weeks, full term 39 0/7 to 40 6/7, late term 41 0/7 to 41 6/7, and postterm from 42 0/7. The distinction matters most for elective delivery, since respiratory problems in newborns are more common in the early term window than at full term.

Read the due date as the midpoint of a normal range rather than an appointment. It marks 40 weeks from the first day of the last period, and a birth two weeks either side of it is ordinary.