Calculate Your Due Date
The average cycle is 28 days, but can range from 20-45 days
Key Pregnancy Milestones
About Due Date Calculation
A pregnancy due date is typically calculated as 280 days (40 weeks) from the first day of your last menstrual period. This calculation assumes a 28-day menstrual cycle.
If your cycle is longer or shorter than 28 days, our calculator adjusts your due date accordingly. For example, if your cycle is typically 35 days, your due date would be calculated as 287 days from your last period.
Only about 5% of babies are actually born on their estimated due date. Most babies are born in the two weeks before or after this date.
Methods of Calculating Due Date
- Naegele's Rule: The most common method, which adds 7 days to the first day of your last period and then counts forward 9 months.
- Conception Date: If you know when conception occurred, your due date is approximately 266 days (38 weeks) from that date.
- Ultrasound Dating: Early ultrasounds can provide the most accurate estimate of your due date, especially if performed in the first trimester.
Frequently Asked Questions
How accurate is a due date calculator?
While due date calculators provide a good estimate, only about 5% of babies are born exactly on their due date. Most are born within two weeks before or after the calculated date.
What if I don't know the date of my last period?
If you can't remember your last period, an early ultrasound (before 20 weeks) can help determine your due date more accurately.
What is the difference between gestational age and fetal age?
Gestational age is counted from the first day of your last period, while fetal age begins at conception (typically 2 weeks later). Medical professionals typically use gestational age.
Why is pregnancy 40 weeks if 9 months is about 36 weeks?
Pregnancy is actually closer to 9.5 months. The 40-week count starts from the beginning of your last period, which is about 2 weeks before conception actually occurs.
What if my cycles are irregular?
If your cycles are irregular, calculating your due date can be less accurate. In this case, an early ultrasound is the best way to establish your due date.