Pregnancy Due Date Calculator: How to Estimate Your Baby's Arrival
Your due date is one of the first milestones in pregnancy. Understanding how it's calculated helps you plan appointments, prepare for your baby, and know what to expect each trimester.
What Is a Due Date Calculator?
A pregnancy due date calculator estimates when your baby is likely to be born based on key dates like your last menstrual period (LMP) or ovulation date. While no calculator can predict the exact day, it provides a reliable 40-week target that helps you and your healthcare provider plan prenatal care.
Three Methods to Calculate Your Due Date
| Method | How It Works | Accuracy |
|---|---|---|
| Last Menstrual Period (LMP) | Add 280 days (40 weeks) to the first day of your last period using Naegele's Rule | Within 2 weeks (assumes regular 28-day cycle) |
| Ovulation / Conception Date | Add 266 days (38 weeks) to your estimated ovulation or conception date | Within 1 week (if ovulation date is known) |
| Ultrasound Dating | Crown-rump length measurement taken between 8-12 weeks of pregnancy | Within 5-7 days (most accurate) |
Step-by-Step: Using the LMP Method
- Find the first day of your last menstrual period.
- Subtract 3 months from that date.
- Add 7 days to the result.
- The new date is your estimated due date.
Example: If your last period started on January 10, subtract 3 months to get October 10, then add 7 days. Your estimated due date is October 17.
When to Use an Ultrasound Instead
If your cycles are irregular, you don't remember your LMP, or you conceived while using contraception, an early ultrasound is the most reliable way to date your pregnancy. First-trimester scans are preferred because the embryo grows at a predictable rate early on.
Try it now: Open the Pregnancy Due Date Calculator →
Understanding Your Timeline
- First trimester (weeks 1-13): Major organ development; morning sickness common.
- Second trimester (weeks 14-27): Baby grows rapidly; you'll feel movement.
- Third trimester (weeks 28-40): Baby gains weight; prepare for delivery.
Remember, only about 5% of babies arrive on their exact due date. Most are born within two weeks before or after the estimate. Use your due date as a guide, not a deadline.
Frequently Asked Questions
How accurate is a due date calculator?
Due date calculators are estimates. Only about 5% of babies are born on their exact due date. Most babies arrive within a two-week window before or after the calculated date. First-trimester ultrasound dating is the most accurate method, typically within 5-7 days.
What if I don't know the date of my last period?
If you don't know your LMP date, an early ultrasound (ideally between 8-12 weeks) is the best way to estimate your due date. Your doctor can measure the crown-rump length to determine gestational age with high accuracy.
Can my due date change after an ultrasound?
Yes. If your first-trimester ultrasound measurement differs from your LMP-based date by more than 7 days, your doctor will typically adjust your due date to match the ultrasound. After the first trimester, due dates are rarely changed.
What is Naegele's Rule?
Naegele's Rule is the standard method for calculating due dates: subtract 3 months from the first day of your last menstrual period, then add 7 days. This assumes a regular 28-day cycle and gives an estimated gestational age of 40 weeks (280 days).