Due Date Calculator
Estimate a pregnancy due date from the first day of the last menstrual period or a conception date.
Sunday, August 15, 2027
Currently about week 0
About this calculator
What Is a Due Date Calculator?
A due date calculator estimates the day a baby is expected to arrive. The most common starting point is the first day of the last menstrual period (LMP), but dates from an ultrasound, a known conception date or an IVF transfer can also be used. Enter the date and you get an estimated due date plus a sense of how far along the pregnancy is. It is a planning aid for parents and a handy reference to bring to the first prenatal appointment.
How the Due Date Is Estimated
The standard method is Naegele's rule: take the first day of the last period, add 280 days (40 weeks), which is the same as adding one year, subtracting three months and adding seven days. It assumes a 28-day cycle with ovulation around day 14. If your cycles are shorter or longer than that, the date shifts by the difference, and a dating ultrasound in the first trimester can refine it. For a conception-date estimate, add 266 days (38 weeks), since conception occurs about two weeks after the LMP.
Gestational Age and Trimesters
Doctors measure pregnancy in gestational age, counted from the first day of the last period — so a woman is already "two weeks pregnant" at conception. Pregnancy is divided into three trimesters: the first runs through week 13, the second from weeks 14 to 27, and the third from week 28 until birth.
Only a Small Share Arrive on the Date
A due date is an estimate, not an appointment. Only about 4% of babies are born on the calculated date, and most arrive within two weeks either side. Babies born between 37 and 42 weeks are considered full term; before 37 weeks is preterm, and after 42 weeks is post-term. Think of the due date as the middle of a window of time rather than a deadline.
What to Do Next
- Share the date with your doctor or midwife to confirm it with an ultrasound.
- Plan prenatal visits, tests and leave from work around the estimated window.
- Track healthy weight changes with your provider's guidance — see also the BMI Calculator.
This tool provides general information, not medical advice. Check anything about your pregnancy with a healthcare professional. To find the number of days between other dates, use the Date Calculator.
Worked example
From the first day of the last menstrual period, the estimated due date is 280 days (40 weeks) later. From a known conception date, it's 266 days (38 weeks) later.
Frequently asked questions
How accurate is an estimated due date?
Only about 1 in 20 babies arrive exactly on the estimated date — it's a statistical average, not a prediction. Full term is generally considered anywhere from 37 to 42 weeks.
Why do LMP and conception dates give different day counts?
The LMP method assumes conception happens about 2 weeks after the last period started (a typical cycle), so 280 days from LMP and 266 days from conception land on close to the same date for an average cycle.
What if I have an irregular cycle?
This calculator uses standard averages and doesn't adjust for cycle length — an ultrasound-based estimate from a healthcare provider is more accurate for irregular cycles.
