Due Date Calculator

About Due Date Calculator

A due date is not the day your baby will arrive. It is the day you reach 40 weeks 0 days. In an Australian study of about 18,700 women who went into labor spontaneously (Khambalia et al., 2013), only about 5% gave birth on their estimated due date, and roughly two thirds gave birth within seven days either side of it. Treat the date as the anchor for scheduling tests and planning, not as a prediction.

This calculator takes the first day of your last menstrual period (LMP) or a conception date, applies Naegele's rule, and shows your due date, how many weeks pregnant you are today, your trimester and progress, and the dates you reach conception, 14, 24, 28, 37, 39, 40, 41 and 42 weeks. If your cycle isn't 28 days, it shifts every date by the difference.

Naegele's rule and the assumptions behind it

The rule is named after the 19th-century German obstetrician Franz Naegele: take the first day of the LMP, add one year, subtract three months and add seven days. The calculator does the same thing with exact date arithmetic by adding 280 days (40 weeks). Because months have different lengths, the two can disagree by a day or two. An LMP of March 1 gives December 8 by hand and December 6 by adding 280 days.

Those 280 days assume a 28-day cycle with ovulation on day 14. In real cycles the luteal phase, from ovulation to the next period, is fairly constant at about 14 days; most of the variation in cycle length happens before ovulation. Someone with a 35-day cycle probably ovulates around day 21, so her due date should be about a week later.

The calculator applies exactly that correction: it shifts the LMP by (average cycle − 28) days to make an 'adjusted LMP' and computes the due date, gestational age and milestones from it. It accepts cycles from 20 to 45 days. If your cycles vary a lot from month to month, an average won't fix the error, and an early ultrasound is the better reference.

How pregnancy weeks are counted

Gestational age is counted from the LMP, not from conception. On the day you conceive you are already '2 weeks 0 days' pregnant, and by the time a home test turns positive you are usually 4 or 5 weeks. Fetal (conceptional) age is two weeks less.

Clinicians write gestational age as completed weeks plus days, such as 12w3d or 12 3/7 weeks, which means you are partway through the 13th week. The calculator uses the same convention. It defines the first trimester as weeks 0–13, the second as 14–27 and the third as 28 onward; some sources draw the first boundary at 12 or 13 weeks, so a day or two of disagreement is normal.

When an ultrasound changes your due date

Naegele's rule fails when the LMP is uncertain or cycles are irregular, so most pregnancies are checked with a first-trimester ultrasound that measures crown-rump length (CRL). The American College of Obstetricians and Gynecologists (ACOG) calls first-trimester ultrasound, up to and including 13 6/7 weeks, the most accurate way to establish or confirm gestational age, with CRL accurate to within about 5–7 days.

Under ACOG's guidance, if the first-trimester ultrasound differs from LMP dating by more than 5 days before 9 weeks, or by more than 7 days from 9 0/7 to 13 6/7 weeks, the due date is changed to match the ultrasound. Once set, it is not changed again because of later scans, since a later size difference more likely reflects growth than wrong dates. If your provider's due date differs from this calculator, go with theirs.

With IVF the conception date is known, which makes dating the most precise of all. The calculator's conception mode subtracts 14 days to build an adjusted LMP and continues as usual. If you know a transfer date instead, convert it to a conception date first by accounting for the embryo's age.

What the milestone dates mean

• 24 weeks: labeled viability in the calculator. It marks the point where a baby born with intensive neonatal care has a chance of survival, not a point of safety, and the threshold used varies between countries and hospitals.
• 37, 39 and 41 weeks: the calculator follows the ACOG definitions used since 2013 and labels 37 weeks as the start of early term (37 0/7–38 6/7), 39 weeks as the start of full term (39 0/7–40 6/7) and 41 weeks as the start of late term (41 0/7–41 6/7). The older convention called everything from 37 weeks “term”, but newborn outcomes, especially breathing problems, differ across those weeks.
• 42 weeks: postterm starts at 42 0/7. Providers usually discuss induction before this point.

Worked examples

How to use it

1. Choose LMP or conception date as the basis.
2. Enter the date and, for LMP mode, your average cycle length (leave 28 if unsure).
3. Read the due date, current week and milestone table. The date must be within the past 300 days.

Frequently asked questions

Notes

This is an estimate. Your medical record's gestational age and due date take precedence.