Pregnancy Due Date Calculator

How to use this calculator#

1. 1
   Enter the first day of your last period, not the last dayGestational age is counted from day one of bleeding. Using the day your period ended pushes the whole estimate four to seven days late, which is enough to change which week of care you are booked into.
2. 2
   Give your usual cycle lengthNaegele's rule assumes 28 days with ovulation on day 14. If your cycles typically run 32 days, ovulation is later and the estimate moves four days back. Enter the length you experience most months, measured from the first day of one period to the first day of the next.
3. 3
   Switch to conception-date mode if you know itIf you conceived through IVF, tracked ovulation with predictor kits, or know the date of conception, that mode adds 266 days instead of 280 — the two-week gap between period and ovulation is already accounted for. For IVF specifically, your clinic's dating from the transfer date and embryo age is the figure to rely on.
4. 4
   Read the gestational age alongside the due dateThe tool shows how many weeks and days along you are today. Antenatal appointments, screening windows and the anatomy scan are all scheduled against gestational age, so that is usually the more practically useful number.
5. 5
   Let an early ultrasound override thisA crown-rump-length measurement before 14 weeks dates a pregnancy more accurately than any calculation. If your scan date and this estimate differ by more than a few days, your provider will redate the pregnancy from the scan — and that clinical date is the one that counts.

The formula#

How the due date is calculated#

The standard method is Naegele's rule: add 280 days, or 40 weeks, to the first day of your last menstrual period. It assumes a 28-day cycle with ovulation on day 14, so this tool adjusts for cycle length by adding the difference between your cycle and 28 days. A 32-day cycle pushes the estimate four days later.

If you know your conception date instead, the due date is 266 days after it, since the two-week gap between the period starting and ovulation is already accounted for. Gestational age is counted from the last period, which is why you are considered two weeks pregnant at the moment of conception.

Trimesters and milestones#

The first trimester runs from week 1 through the end of week 13, the second from week 14 through week 27, and the third from week 28 until birth. A pregnancy is early term at 37 weeks, full term from 39 weeks, and post-term after 42 weeks, at which point most providers discuss induction.

Common milestones fall at predictable points: the dating ultrasound around weeks 8 to 12, the anatomy scan near week 20, glucose screening around weeks 24 to 28, and viability considered from about week 24. Your provider will set the exact schedule for your pregnancy.

How reliable is an estimated due date#

Only about 4% of babies arrive on their estimated due date, and roughly 80% are born within two weeks either side of it. An early ultrasound, particularly a crown-rump-length measurement before 14 weeks, dates a pregnancy more accurately than last-period arithmetic and will override this estimate if the two disagree by more than a few days.

This calculator gives an estimate for general information only. It is not medical advice, not a diagnosis, and not a substitute for antenatal care. Confirm your dates with a midwife, obstetrician or GP, and contact them straight away about bleeding, severe pain, reduced fetal movement or any concern about your pregnancy.

Worked examples#

Reference tables#

Common mistakes#

• Counting from the wrong day of the periodDay one is the first day of full bleeding, not spotting and not the day bleeding stopped. Getting this wrong by five days moves the due date by five days and can put screening windows — several of which are only a few weeks wide — out of alignment.
• Ignoring a long or irregular cycleNaegele's rule bakes in ovulation on day 14. Someone with a 35-day cycle ovulates around day 21, so the unadjusted estimate is a week early. On genuinely irregular cycles the arithmetic has no reliable ovulation day to work from at all.
• Treating the due date as an appointmentAround 4% of babies are born on their estimated due date and roughly 80% arrive within two weeks either side. Planning leave, travel or childcare around the exact date, rather than around a window, sets up an avoidable problem.
• Preferring the calculated date over an ultrasoundA first-trimester crown-rump-length measurement is the most accurate dating method available, and clinical guidance is that it supersedes LMP-based dating when the two disagree beyond a defined margin. Redating is a normal part of care, not an error.

Frequently asked questions#

Key terms#

LMP (last menstrual period)

The first day of the most recent menstrual bleed. The anchor point for gestational dating in the absence of an ultrasound.

Gestational age

How far along a pregnancy is, counted in completed weeks and days from the LMP. It runs about two weeks ahead of fetal (conceptional) age.

EDD / EDC

Estimated due date, sometimes written as estimated date of confinement. The date 280 days after LMP, or 40 completed weeks of gestation.

Crown-rump length (CRL)

The measurement from the top of the fetal head to the bottom of the torso on ultrasound. Before 14 weeks it dates a pregnancy more accurately than any other method.

Naegele's rule

The convention of adding 280 days to the LMP, traditionally expressed as subtracting three months and adding seven days and a year.

Trimester

One of three conventional thirds of pregnancy, most commonly defined as weeks 1–13, 14–27 and 28 onwards. A scheduling convention rather than a biological boundary.

Sources#