How your due date is calculated
Doctors and midwives count pregnancy from the first day of your last menstrual period (LMP), not from the day you conceived. A full-term pregnancy is about 280 days, or 40 weeks, from that date. That means at the moment of conception you're already counted as about 2 weeks pregnant.
You can work it out by hand with Naegele's rule: take the first day of your last period, subtract 3 months, add 7 days, and add 1 year. A last period starting January 1 gives a due date of October 8 (in a non-leap year). If your cycles are longer or shorter than 28 days, ovulation happens later or earlier, so this calculator shifts your due date by the difference.
How accurate is a due date?
A due date is an estimate, not an appointment. Few babies arrive on the exact day; most are born within about two weeks of it. An ultrasound in the first trimester is the most accurate way to date a pregnancy, so if yours gives a different date, go with the one your provider sets. Since 2013, the American College of Obstetricians and Gynecologists has defined full term as 39 weeks 0 days through 40 weeks 6 days.
Planning ahead for baby costs
Your due date is also a savings deadline. If you'd like to put money aside for baby gear, medical bills or unpaid leave, divide your goal by the weeks you have left. Saving $2,000 over 30 weeks is about $67 a week. The Savings Goal Calculator works it out for any amount and date.
This calculator is for general information only and isn't medical advice. Always follow the due date and care plan from your doctor or midwife.
Frequently asked questions
How is a due date calculated?
A due date is usually 280 days (40 weeks) after the first day of your last menstrual period, assuming a 28-day cycle. A quick way to do it by hand is Naegele's rule: take the first day of your last period, subtract 3 months, add 7 days and add 1 year.
How accurate is my due date?
A due date is an estimate. Few babies arrive on the exact day, and most are born within about two weeks of it. An ultrasound in the first trimester is the most accurate way to date a pregnancy, so your provider may adjust your due date after one.
How many weeks pregnant am I?
Pregnancy is counted from the first day of your last period, not from conception. So at the moment of conception you are already about 2 weeks pregnant, and a pregnancy lasts about 40 weeks in total.
How do I calculate my due date from my conception date?
Add 266 days (38 weeks) to the date you conceived. This is the same as 280 days from your last period for someone with a 28-day cycle, since ovulation usually happens about 2 weeks after a period starts.
How do I calculate an IVF due date?
For a day-5 embryo (blastocyst) transfer, add 261 days to the transfer date. For a day-3 embryo transfer, add 263 days.
When does each trimester start?
The first trimester runs from week 1 through the end of week 13. The second trimester runs from week 14 through the end of week 27, and the third trimester starts at week 28 and lasts until birth.