Calculator
How it works and when to use it
The estimated due date is 280 days (40 weeks) after the first day of the last menstrual period, known as Naegele's rule, and when your cycle is longer or shorter than 28 days the date moves by the same difference, assuming the time from ovulation to the next period stays about the same (around 14 days).
You can type the last period in the Gregorian or the Hijri calendar: choose «Hijri (Umm al-Qura)» as the calendar and the calculator turns the Hijri date into its Gregorian day by the Umm al-Qura calendar used in Saudi Arabia, then shows the due date in both, the Hijri date is a conversion and may differ by a day from the moon sighting.
A pregnancy is at term from 37 weeks, the date is an estimate, and your doctor confirms the age of the pregnancy with a first-trimester ultrasound, the most accurate way to date it.
Reference: Naegele's rule: EDD = LMP + 280 days, adjusted by (cycle length − 28) days; ACOG Committee Opinion No. 700, Methods for Estimating the Due Date (2017); term definitions: ACOG Committee Opinion No. 579 (2013)
Months of pregnancy in weeks
| Month | Weeks | Trimester |
|---|---|---|
| Month 1 | 1 to 4 | First |
| Month 2 | 5 to 8 | First |
| Month 3 | 9 to 13 | First |
| Month 4 | 14 to 17 | Second |
| Month 5 | 18 to 22 | Second |
| Month 6 | 23 to 27 | Second |
| Month 7 | 28 to 31 | Third |
| Month 8 | 32 to 35 | Third |
| Month 9 | 36 to 40 | Third |
Splitting the weeks into months is approximate and a common convention, since a month is not a fixed number of weeks, the trimesters follow the American College of Obstetricians and Gynecologists (ACOG): the first to the end of week 13, the second from 14 to the end of 27, the third from 28.
Frequently asked questions
From which day are the weeks of pregnancy counted?
From the first day of the last period, not from conception, so in a usual 28-day cycle a pregnancy is already about two weeks along on the day of conception.
What if my cycle is irregular or I do not remember the date?
Dating by the last period is then less accurate, and the doctor relies on measuring the baby on a first-trimester ultrasound.
Can the doctor change my due date after the ultrasound?
Yes, when the ultrasound and the last period differ by more than a set number of days, which medical guidelines set according to how far along the pregnancy is at the scan.
When should I start seeing a doctor?
Early, in the first trimester, to confirm and date the pregnancy and do the first tests.
Tools you may also need
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- Period calculator The dates your next periods start and end.
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- Ideal weight calculator The lowest and highest healthy weight for your height, and what to lose or gain.
- Body fat percentage calculator Body fat from a tape measure, and fat mass in kilograms.
Clinic software for this specialty
Try Clinicato for your clinic
These calculators are free here for everyone, and in Clinicato 49 medical tools work inside the patient's file with the "Medical tools" add-on: filled from the visit and the vital signs, with the result added to the visit's plan, next to appointments, prescriptions and invoices.