How Do I Calculate My Due Date of Pregnancy?

There are different ways to calculate your due date, which is based on counting from the first day of your last menstrual period, your conception date, or your IVF transfer date. It can also be estimated based on certain measurements or milestones.

When you’re trying to conceive and finally get a positive pregnancy test, how do you know your due date? 

Most pregnancies last about 40 weeks (or 38 weeks from conception) and the estimated due date is the date that the spontaneous onset of labor is expected to occur. It’s important to remember this is an estimated date, and only about four percent of babies are born on the estimated due date. 

There are several ways to calculate the due date of pregnancy: 

• First day of the last period
  • The due date may be estimated by adding 280 days (or roughly 40 weeks) to the first day of the last menstrual period
  • The first day of the last menstrual period is how most obstetricians estimate a baby’s due date
  • “Pregnancy wheels” (also called gestation calculators) also use this method
  • The accuracy of this method depends on an accurate recall by the mother, assumes regular 28-day menstrual cycles, and that ovulation and conception occurs on day 14 of the cycle
  • Subtracting three months from the first day of the last period and adding seven days may also be used 
• Conception date
  • For those who use ovulation test strips or track ovulation symptoms, the conception date may be used to calculate the due date of pregnancy by adding 266 days (or roughly 38 weeks) to the conception date
• In vitro fertilization (IVF) transfer date
  • For those who conceived through IVF transfer, the due date of pregnancy may be calculated using the IVF transfer date
  • Embryo transfers usually occur either three days or five days after egg retrieval and fertilization
  • For those who had a day-three transfer, add 263 days from the transfer date to calculate the due date, and for those who had a day-five transfer, add 261 days
• Other ways to help determine the due date include: 
  • An early ultrasound 
  • Pregnancy milestones such as: 
    • The first time the baby’s heartbeat is heard (usually around week 9 or 10) 
    • Feeling fetal movement for the first time (on average between 18 and 22 weeks
  • Fundal height
    • The measurement from the pubic bone to the top of the uterus
  • The size of the uterus