Pregnancy Due Date Calculator
Estimated due date, weeks pregnant, trimester and milestone dates.
Use via API
Estimated pregnancy due date (40 weeks) from the last period, conception date or IVF transfer, with milestone dates and the gestational age on a given day. Send JSON, get JSON back.
POST https://pithlab.com/api/v1/tools/due-date-calculator
curl https://pithlab.com/api/v1/tools/due-date-calculator \
-H "Authorization: Bearer $PITHLAB_KEY" \
-H "Content-Type: application/json" \
-d '{"method":"lmp","date":"2026-03-01","asOf":"2026-10-20"}'About the Due Date Calculator
Estimate a pregnancy due date from the last period, conception date or IVF transfer, with how many weeks along you are, the trimester and key dates.
How to use the Due Date Calculator
- Choose what you know: the first day of your last menstrual period (LMP), the conception date, or an IVF embryo transfer date.
- Enter the date. For LMP, set your usual cycle length; for IVF, choose a day-3 or day-5 embryo transfer.
- Read the estimated due date, how far along you are today (or on any date you pick), the trimester and the dates of key milestones.
Examples
Input
LMP 2026-10-02, 28-day cycle, as of 2026-10-19
Output
Due 2027-07-09 (Friday, 9 July 2027); 2 weeks 3 days pregnant on 2026-10-19, first trimester
Input
LMP 2026-03-10, 32-day cycle
Output
Due 2026-12-19 (Saturday, 19 December 2026)
Input
Conception 2026-08-20
Output
Due 2027-05-13 (Thursday, 13 May 2027)
Input
Transfer 2026-09-01, day-5 embryo
Output
Due 2027-05-20 (Thursday, 20 May 2027)
Frequently asked questions
How is the due date calculated?
From the last period it uses Naegele's rule: 280 days (40 weeks) after the first day of your last period, moved later or earlier by however much your cycle is longer or shorter than 28 days. From conception it adds 266 days. For IVF it adds 266 days minus the embryo's age at transfer: 263 days for a day-3 transfer and 261 days for a day-5 transfer.
How accurate is a due date?
It's an estimate. Only around 4% of babies arrive on their due date, and most are born within a week or two either side. An early ultrasound measurement often gives a more reliable date, and your midwife or doctor may adjust it.
Why am I “2 weeks pregnant” at conception?
Pregnancy is conventionally counted from the first day of the last period, about two weeks before ovulation and conception. So gestational age is about two weeks more than the time since conception.
How are trimesters split?
This tool uses the common split: first trimester up to 13 weeks 6 days, second from 14 weeks to 27 weeks 6 days, and third from 28 weeks. Some sources draw the lines a week differently.
Is this medical advice?
No. It's a calendar estimate for planning. Always follow the dates and advice from your own healthcare provider.