Due Date Calculator & Pregnancy Timeline
Select a method—LMP, conception/IUI, IVF (day‑3 or day‑5), or ultrasound—enter one date, and we’ll calculate your EDD, how many weeks pregnant you are, trimester dates, EWC, maternity leave week, and baby size this week. Save to calendar or print. Nothing is stored.
Select Method
Use the date your last period started. We’ll adjust for your cycle length.
Export & print
Calendar export adds one event on your due date (EDD). All other key dates are included in the event notes.
How we calculate your due date
By convention, due dates are estimated as 280 days after the first day of your last menstrual period (LMP). If your cycle is not 28 days, we adjust by the difference. If you know your conception / IUI date, we add 266 days. If you had IVF, we add 261 days for day-5 transfers or 263 days for day-3. Ultrasound dating in the first trimester is most accurate and will override LMP if there’s a mismatch.
What is the EWC?
EWC stands for Expected Week of Childbirth — the Sunday–Saturday week that contains your due date. In the UK, statutory maternity leave can begin from 11 weeks before your EWC.
How to calculate your pregnancy due date
LMP due date calculator
Your due date is traditionally estimated as 280 days after the first day of your last menstrual period (LMP). If your cycle isn’t 28 days, our calculator adjusts by the difference.
Conception or IUI due date
If you know the day of conception or intrauterine insemination (IUI), add 266 days to get your estimated due date.
IVF due date (day‑3 vs day‑5)
For IVF, due dates are based on embryo age at transfer: add 263 days for day‑3 embryos or 261 days for day‑5 blastocysts. The tool handles this automatically.
Ultrasound due date
An early scan is the most accurate way to date a pregnancy. We use the gestational age at scan to project your EDD: EDD = scan date + (280 − gestational days at scan).
How many weeks pregnant am I?
Pregnancy is counted from your LMP, not conception. That means you’re around two weeks “pregnant” at conception. Our calculator shows weeks + days today, your trimester, and your week number.
Due date calculator FAQs
How accurate is a due date?
It’s an estimate. Most babies arrive between 37 and 42 weeks. First-trimester ultrasound is the most accurate way to date a pregnancy.
Why is my due date different after a scan?
Ultrasound measures the baby directly (crown–rump length early on), which is more reliable than counting from your period. Clinicians typically use scan dates when there’s a significant difference.
What if I have irregular cycles?
Use the ultrasound or conception/IUI method if possible. If using LMP, the calculator adjusts for your average cycle length, but an early scan is best for accuracy.
When can I start maternity leave in the UK?
You can usually start from 11 weeks before your Expected Week of Childbirth (EWC). Your employer may have additional policies—check your contract.
How many weeks is a pregnancy?
A full‑term pregnancy is around 40 weeks. Babies are considered full term between 37–42 weeks.
Does IVF change how due dates are calculated?
Yes. IVF due dates use the embryo age at transfer: day‑3 + 263 days or day‑5 + 261 days. This accounts for development before transfer.
Can my due date change after an ultrasound?
Yes. If the scan date differs significantly from LMP dating, clinicians usually use the scan because it measures the baby directly.
What if my periods are irregular?
Use the ultrasound or conception/IUI method if possible. If using LMP, set your average cycle length—the calculator adjusts the EDD accordingly.