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

After a positive pregnancy test, the first question most expectant parents ask is: “When is my due date?” While the average pregnancy lasts about 40 weeks (280 days) from the first day of your last menstrual period, every pregnancy is unique. A pregnancy calculator uses established medical guidelines to give you an estimated due date, current gestational age, and trimester breakdown — helping you plan prenatal appointments, maternity leave, and preparations.

Use the free Pregnancy Calculator at Today Calculator to estimate your due date instantly.

How Due Dates Are Calculated: Naegele’s Rule

Obstetricians use a standard method called Naegele’s Rule:

1. Take the first day of your last menstrual period (LMP)
2. Add 7 days
3. Subtract 3 months
4. The result is your estimated due date

Example: LMP = March 1, 2026 → Add 7 days = March 8 → Subtract 3 months = December 8, 2026

This formula assumes a regular 28-day cycle with ovulation on day 14. If your cycle is longer, shorter, or irregular, the estimate adjusts. The pregnancy calculator also accepts a conception date for greater accuracy.

Pregnancy Timeline: Three Trimesters at a Glance

Trimester	Weeks	What Happens	Key Appointments
First	1–12	Conception, heartbeat begins, major organs form, morning sickness common	First prenatal visit (weeks 8–10)
Second	13–26	Fetal movement felt, anatomy visible on ultrasound, growth accelerates	Anatomy scan (weeks 18–22)
Third	27–40	Rapid weight gain, lungs mature, baby settles into position	Weekly checkups (weeks 36+)

What Affects Due Date Accuracy?

• Irregular cycles: Cycles longer than 35 days or shorter than 21 days shift ovulation timing.
• Early ultrasound: Crown-rump length measurements between weeks 7–13 provide the most accurate dating.
• Multiple pregnancies: Twins are typically delivered around 36–37 weeks.
• IVF pregnancies: Due dates are calculated from the embryo transfer date, not LMP.
• Only 4% of babies arrive on their exact due date — treat it as a target window, not a guarantee.

How to Use the Pregnancy Calculator

1. Enter your LMP date or conception date.
2. The calculator instantly shows your estimated due date, current gestational age (weeks + days), and current trimester.
3. Use the results to schedule prenatal appointments, plan maternity leave, and prepare for your baby’s arrival.

Share your results with your healthcare provider — they will confirm or adjust based on clinical evaluation and ultrasound. Get started now with the Pregnancy Calculator.