How is my due date calculated?
Pregnancy due dates are calculated by adding 40 weeks (280 days) to the first day of your last menstrual period (LMP). This method is used because pinpointing the exact conception date is difficult, sperm can survive up to 5 days, and ovulation timing varies.
Here's how it works
- 1Start with the first day of your last period
- 2Add 40 weeks (280 days)
- 3Use the result as an estimated due date
Example: If your last period started on January 1, your estimated due date would be around October 8.
This calculation assumes a 28-day cycle with ovulation around day 14. Different cycle lengths or ovulation dates can shift the estimate. Your healthcare provider can check the date with an ultrasound.
Approximate birth month
This table gives a rough birth month based on conception about 38 weeks before the due date. Exact dates can fall in a neighboring month.
Your care team will confirm your estimated due date and discuss monitoring if pregnancy continues beyond it.
How accurate is a pregnancy due date?
A due date estimates when you reach 40 weeks of pregnancy. It cannot predict the day labor will start. Dating from the last period also depends on how closely ovulation timing matches the calculation.
An early ultrasound can help confirm or revise the estimate. Use the date agreed with your care team for appointments and pregnancy care.
Why your due date matters for prenatal care
Your care team uses the estimated due date to:
- Schedule important tests and screenings at the right time
- Monitor your baby's growth and development appropriately
- Determine if your pregnancy is full-term, preterm, or post-term
- Plan timing for induction if needed
- Follow pregnancy milestones
Bring your period dates and any questions about timing to your first pregnancy appointment.
