PonyatnoHow it's calculated
From your last period — Naegele's rule: due date = first day of last period + 280 days (40 weeks). It assumes a 28-day cycle. If your cycle is longer or shorter, ovulation shifts, so the calculator moves the date by the same difference: EDD = LMP + 280 + (cycle length − 28).
From conception: EDD = conception date + 266 days (38 weeks). From ultrasound: the gestational age in the report is counted back from the scan day — EDD = scan date + (280 − gestational age in days). From IVF: day-3 embryo transfer + 263 days, day-5 (blastocyst) transfer + 261 days, as recommended by ACOG.
Pregnancy is dated from the last menstrual period (with other methods — from a calculated date 14 days before conception) up to today, in completed weeks and days. Trimesters: first — up to 13 weeks 6 days, second — 14 to 27 weeks 6 days, third — from 28 weeks.
What to know
Which method is most accurate
According to the American College of Obstetricians and Gynecologists (ACOG), the most accurate way to date a pregnancy is a first-trimester ultrasound (up to 13 weeks 6 days) measuring crown–rump length. If the dates from your period and the early scan differ noticeably, your doctor or midwife usually goes by the scan and does not change the due date later. Later scans are less reliable for dating because babies grow at different rates.
With IVF the transfer date is known exactly, so dating from it is the most reliable of all.
Why the due date is not exact
Dating from your period assumes you ovulated on the «expected» day, but ovulation can shift by several days. Not everyone remembers the date of their last period exactly, and with irregular cycles, after stopping hormonal contraception or while breastfeeding, the period date says little about how far along you are.
Even with accurate dating, only about 4–5% of babies are born on their due date. Birth within a few weeks around it is considered term — that is normal.
Why you are «further along» than since conception
Gestational age is counted from the first day of your last period, and conception usually happens about two weeks later. So at «5 weeks pregnant» the embryo is about three weeks old. Doctors, midwives and scan reports all use this way of counting.
This calculator is a reference tool, not a diagnosis. Laboratory ranges differ, and the result depends on your health, medicines and other tests. Discuss it with your doctor.
FAQ
How do I calculate my due date from my last period?
Add 280 days (40 weeks) to the first day of your last period — that is Naegele's rule. A quick shortcut: subtract 3 months and add 7 days. If your cycle is not 28 days, the date shifts by the difference, which this calculator takes into account.
Which is more accurate: dating by period or by ultrasound?
An early, first-trimester ultrasound. If the two differ significantly, your doctor will usually go by the scan.
How many babies are born on their due date?
About 4–5%. Most are born within roughly two weeks either side, which is perfectly usual.
Which trimester am I in?
First trimester — up to 13 weeks 6 days, second — 14 to 27 weeks 6 days, third — from 28 weeks until birth. The calculator shows your trimester next to your gestational age.
How is the due date calculated after IVF?
From the transfer date: for a day-3 embryo, EDD = transfer date + 263 days; for a day-5 embryo, + 261 days. Gestational age is still counted the usual way, as if from a last period.
Sources
Formulas and cut-offs follow clinical guidelines — see sources below.
- ACOG Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150–e154.
- Naegele's rule: due date = first day of the last menstrual period + 280 days (for a 28-day cycle).