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Pregnancy Due Date Calculator
Calculate your baby's due date based on your last menstrual period (LMP). Track pregnancy weeks and trimester milestones.
First Day of Last Period
Understanding Your Pregnancy Timeline
Pregnancy is calculated from the first day of your last menstrual period (LMP), not from conception. This is because ovulation and conception dates are harder to pinpoint, while LMP is a specific, known date. A full-term pregnancy lasts about 40 weeks (280 days) from your LMP.
đź’ˇ Expert Tip from Sarah Jenkins, RD
Only about 5% of babies are born on their exact due date. Most arrive within 2 weeks before or after. Think of your due date as a "due month" instead. That said, eating well throughout pregnancy—plenty of folate, iron, and DHA—can support a healthy full-term delivery.
What to Expect By Trimester
- First Trimester (Weeks 1-12): Morning sickness, fatigue, and rapid fetal development. All major organs form during this period.
- Second Trimester (Weeks 13-27): The "honeymoon phase"—energy returns, baby bump shows, and you might feel baby's first kicks around week 20.
- Third Trimester (Weeks 28-40): Baby gains weight rapidly, and you may experience back pain, swelling, and Braxton Hicks contractions.
⚠️ Common Mistake: Confusing Conception Date with LMP
If you conceived on July 1st, your LMP was likely around June 15th (2 weeks earlier). Doctors use LMP because it's a fixed date you can remember, while conception happens around ovulation (typically day 14 of your cycle). Always use LMP when calculating due dates for consistency with medical records.
How Due Dates Are Calculated
The standard method is Naegele's Rule: LMP + 280 days (or LMP + 1 year - 3 months + 7 days). This assumes a 28-day menstrual cycle with ovulation on day 14. If your cycles are irregular, your doctor may use an ultrasound to estimate a more accurate due date, especially in the first trimester.
When to Expect Key Milestones
- Week 6-8: First ultrasound (transvaginal) to confirm pregnancy and heartbeat
- Week 12: First trimester screening (NT scan + blood test)
- Week 20: Anatomy scan (the "big ultrasound" where you can learn baby's sex)
- Week 28: Glucose test for gestational diabetes
- Week 36: Group B Strep test, weekly checkups begin
References
- American College of Obstetricians and Gynecologists. How Your Fetus Grows During Pregnancy.
- Mayo Clinic. Pregnancy Week by Week.
Reviewed by Sarah Jenkins, RD
Registered Dietitian & Maternal Nutrition Specialist
Sarah provides evidence-based prenatal nutrition guidance to support healthy pregnancies.
Frequently Asked Questions
How accurate is this due date calculator?
It's an estimate based on the standard 280-day rule (Naegele's Rule). Only 5% of babies are born on their exact due date. Most are born within two weeks of this date. An ultrasound in the first trimester is the most accurate way to date a pregnancy.
Can I calculate my due date if I have irregular periods?
If your cycles are irregular, the LMP method might be off. You can try estimating based on your average cycle length if you know it, but an early ultrasound is your best bet for accuracy.
When should I see a doctor?
You should schedule your first prenatal appointment as soon as you get a positive home pregnancy test. Most doctors will see you around 8 weeks for your first ultrasound.