How Doctors Calculate Due Dates
It might seem strange, but your pregnancy "clock" actually starts before you are pregnant. Doctors calculate your due date from the first day of your Last Menstrual Period (LMP), not the day of conception.
This method is known as Naegele's Rule. It assumes a standard 28-day cycle and that ovulation occurs exactly 14 days after your period starts.
The "Phantom" Two Weeks
Because the clock starts at your last period, you aren't actually pregnant during weeks 1 and 2—your body is just preparing to release an egg.
- Week 1-2: Menstruation and Ovulation prep. (Not pregnant yet).
- Week 3: Fertilization happens (Conception).
- Week 4: The fertilized egg implants in the uterus. You miss your period.
This is why when you get a positive test after missing your period, doctors say you are already 4 weeks pregnant, even though the baby has only existed for 2 weeks.
Frequently Asked Questions
How accurate is this date? ▼
Only about 5% of babies are born on their exact due date. It is better to think of it as a "Due Month." Most healthy babies arrive anywhere between 37 and 42 weeks.
What if my cycle isn't 28 days? ▼
This calculator adjusts for that! If your cycle is longer (e.g., 35 days), your ovulation likely happened later, pushing your due date back. If your cycle is shorter, your due date will be earlier.