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CALCPARK

Pregnancy Calculator

Due date from your last period (Naegele’s rule, cycle-adjusted), conception date or IVF transfer, plus weeks pregnant, trimester, milestone dates and a countdown.

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How to use it

  1. Choose what you know: the first day of your last period, the conception date, an IVF transfer date (with the embryo age) or a due date your clinic already gave you.
  2. Enter that date. For the last-period method also enter your average cycle length; 28 days is the default.
  3. Read the due date, the current week and day, the trimester, the countdown and the progress bar.
  4. Check the milestones table for the dates of typical scans and tests; the next one is highlighted. Change “Progress on” to see any other date.
  5. Copy the summary or the link to keep or share the dates.

Example

Last period started on 1 January 2026, 28-day cycle: due date 1 January + 280 days = 8 October 2026. On 12 March 2026 that is 70 days from the start: 10 weeks 0 days, first trimester, 25 % complete, 210 days to go, estimated conception 15 January. With a 35-day cycle the due date moves a week later to 15 October, because ovulation came about a week later. The same due date follows from a conception date of 15 January (+ 266) or a day-5 blastocyst transferred on 20 January (+ 261).

Formulas, definitions and limits

due (LMP)          = LMP + 280 days + (cycle length − 28)
due (conception)   = conception + 266 days
due (IVF)          = transfer + 266 − embryo age (263 for day 3, 261 for day 5)
gestational day 0  = due − 280 days
weeks pregnant     = floor(days since day 0 ÷ 7), remainder = days
trimester          = 1 until 13w6d, 2 from 14w0d, 3 from 28w0d

Naegele’s rule assumes ovulation on day 14 of a 28-day cycle; the cycle-length adjustment shifts that assumption but cannot capture month-to-month variation, which is why ACOG recommends confirming the due date with a first-trimester ultrasound and redating when the scan differs by more than 5 days before 9 weeks or more than 7 days from 9 to 13 weeks. The IVF offsets treat the embryo’s age at transfer as days already elapsed since fertilisation. Milestone weeks are typical schedules (NHS and ACOG); your own clinic’s timing may differ. The calculator works on calendar dates only and ignores time zones and the time of day.

Sources: ACOG Committee Opinion 700 – Methods for estimating the due date (2017), ACOG Committee Opinion 579 – Definition of term pregnancy, NHS – Your antenatal care schedule, Jukic et al. 2013, Human Reproduction – Length of human pregnancy and contributors to its natural variation.

The dates are estimates for information only and are not medical advice. Your midwife or doctor sets the official due date, usually from an early ultrasound, and decides on any tests or interventions.

How reliable is the due date?

In a study that pinpointed ovulation with hormone measurements, natural pregnancies lasted a median of 268 days from ovulation with a spread of 37 days between the shortest and longest after excluding preterm births – so a five-day difference between a calculated and a scan-based due date is normal. The number that matters more than the date itself is the window: 37 to 42 weeks covers the great majority of births.

Questions

How many weeks pregnant am I?
Count the days since the first day of your last period and divide by 7. The tool does this for today or any date you choose, and it reconstructs the same count from a conception, IVF or due date.
Why is the due date 40 weeks if pregnancy lasts 38?
Gestational age is counted from the last period, about two weeks before conception, because that date is known. Fertilisation to birth is about 38 weeks (266 days); from the last period it is 40 weeks (280 days).
What if my ultrasound gives a different date?
An ultrasound before 14 weeks is the most accurate method. If it differs from the period-based date by more than 5–7 days, clinics use the scan date. Enter it in the "Known due date" mode to follow the weeks from there.