Due date calculator
Enter the first day of your last period, or switch to conception, IVF transfer or ultrasound dating. You'll get your estimated due date, your likely delivery window and the dates of each trimester.
28 days is the textbook default. Longer cycles usually mean later ovulation, so a later due date.
Pick a date to see your due date.
How a due date is worked out
Day 1 is your period, not conception
Pregnancy is dated from the first day of your last menstrual period (LMP), because that is the one date most people know. For the first two "weeks" you aren't pregnant yet.
Step-by-step pictures like this are the fastest way to see why a rule works, not only what it says. For the same reason, ahaboo builds explainers you can drag and adjust, including one where you move the Moon around Earth to see its phases.
Which dating method should you use?
- Last menstrual period (LMP): the default. Works best with regular cycles and a date you're sure of.
- Conception date: if you tracked ovulation or know the date. Due date = conception + 266 days.
- IVF transfer: the most precise, because embryo age is known. See the IVF due date calculator for more detail.
- Ultrasound: a first-trimester crown-rump length measurement is accurate to about ±5–7 days, and is what ACOG uses to confirm or change a due date.
Already know your due date? See how many weeks pregnant you are or read about your baby in pregnancy week by week.
Frequently asked questions
How accurate is a due date calculator?
It gives the same estimate your provider starts with, but only about 4–5% of babies are born on their due date. Around 80% arrive within two weeks either side. An early ultrasound (before 14 weeks) is the most accurate way to date a pregnancy, and ACOG recommends using it when it differs from the LMP date by more than 5–7 days.
Why is pregnancy 40 weeks if babies grow for 38?
Pregnancy is counted from the first day of your last period, roughly two weeks before you ovulate and conceive. So 40 weeks of gestational age equals about 38 weeks since conception.
My cycles are longer than 28 days. Does that change my due date?
Usually yes. Ovulation tends to happen about 14 days before your next period, so a 35-day cycle means ovulating around day 21 and a due date about a week later than the standard calculation. Set your cycle length in the calculator to adjust for this.
How is an IVF due date calculated?
Because the embryo's age is known exactly, the date is precise: for a day 5 blastocyst transfer, the due date is 261 days after transfer; for day 3, 263 days; for day 6, 260 days. Use the IVF option above or our dedicated IVF due date calculator.
Can my due date change?
Your provider may adjust it after the dating scan if the baby's measurements point to a date that differs from your LMP date. After that first-trimester adjustment, due dates are not normally changed, even if the baby later measures big or small.
Sources: ACOG Committee Opinion 700, "Methods for Estimating the Due Date" (2017); Jukic et al., Human Reproduction (2013) on natural variation in pregnancy length.