Pregnancy Due Date Calculator: How to Estimate Your Due Date and Track Each Trimester

Finding out you are expecting brings a flood of questions, but the first one is almost always: “When is my due date?” While only about 5% of babies arrive exactly on their estimated due date, having a reliable estimate helps you schedule prenatal visits, plan maternity leave, and track your baby’s development week by week.

Our online calculators can help you track important dates and milestones throughout your pregnancy journey.

How Due Dates Are Calculated

The most common method is Naegele’s Rule, which has been used for over 150 years. Here is how it works:

• Start with the first day of your last menstrual period (LMP)
• Add 280 days (40 weeks)
• That is your estimated due date

This formula assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, your actual due date may shift.

Alternative Methods

Method	How It Works	Accuracy
Last Menstrual Period	Add 280 days from LMP	±2 weeks
First Ultrasound	Crown-rump length at 8-13 weeks	±5-7 days
Conception Date (IVF)	Add 266 days from conception	±3 days
Fundal Height	After 20 weeks, measure from pubic bone	±2 weeks

Ultrasound dating in the first trimester is considered the most accurate method. If there is a significant discrepancy between LMP-based dates and ultrasound measurements, providers usually go with the ultrasound.

Pregnancy Timeline: What to Expect Each Trimester

First Trimester (Weeks 1-13)

• Weeks 4-6: The embryo implants. A home pregnancy test can detect hCG levels.
• Weeks 8-10: Major organs form. The heart begins beating.
• Weeks 12-13: The risk of miscarriage drops significantly. Morning sickness often peaks.

Second Trimester (Weeks 14-27)

• Weeks 16-20: You may feel the baby move for the first time (quickening). The anatomy scan happens around week 20.
• Weeks 24-26: The fetus reaches viability — it has a chance of survival with medical support outside the womb.
• Week 27: The baby can open its eyes and respond to sound.

Third Trimester (Weeks 28-40)

• Weeks 32-36: The baby settles into a head-down position. Braxton Hicks contractions may begin.
• Weeks 37-40: Full term. Most babies arrive between 38 and 42 weeks. Only 5% arrive on the exact due date.

Factors That Can Change Your Due Date

• Irregular cycles: If you have PCOS or irregular periods, ovulation may happen later than day 14, shifting the due date.
• Multiple pregnancy: Twins are typically delivered around 36-37 weeks.
• First pregnancy: First-time mothers tend to go slightly longer on average than subsequent pregnancies.
• Maternal age: Women over 35 are more likely to have due dates adjusted based on growth scans.

Remember: your due date is an estimate, not a deadline. Use a reliable pregnancy calculator alongside your healthcare provider’s guidance for the best results.