Pregnancy Due Date Calculator
Find out your exact EDD, current trimester, and countdown to baby using standard medical algorithms.
Short Answer: Your estimated due date (EDD) is mathematically calculated by adding exactly 280 days (40 weeks) to the first day of your last menstrual period (LMP).
How it works
Uses Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period. This is the standard method used by OB/GYNs.
The Science of Pregnancy Dating
Here's the thing: Most women think their pregnancy lasts for nine months, starting from the day they conceived. But modern obstetrics doesn't calculate it that way. Medical professionals track pregnancy in weeks, and the clock starts ticking long before you actually get pregnant.
Your due date is anchored to the first day of your Last Menstrual Period (LMP). Because the exact moment of ovulation and fertilization is impossible to know for certain in a natural conception, doctors use the LMP as a reliable, objective starting line.
Naegele's Rule (The Standard Medical Algorithm)
Named after Franz Karl Naegele, an 19th-century obstetrician, this formula mathematically equates to adding exactly 280 days to your LMP.
Manual Step: Calculating Your Due Date by Hand
If you don't have a calculator, here is how doctors do the mental math using Naegele's Rule.
Why is the Due Date Just an "Estimate"?
The EDD is not a deadline. In fact, only about 4% to 5% of babies are actually born precisely on their calculated due date.
The 280-day rule assumes that every woman has a perfect 28-day menstrual cycle and ovulates exactly on day 14. If you have a longer cycle (e.g., 35 days), you likely ovulate later, meaning your true gestational age is slightly younger than the standard LMP math suggests.
Ultrasound Dating vs. LMP Dating
Because of cycle variations, an early dating ultrasound is considered the gold standard. During a first-trimester scan (usually between 7 and 12 weeks), the technician measures the fetus from the top of its head to its bottom (Crown-Rump Length, or CRL).
- If the ultrasound date is within 5 days of your LMP date, doctors usually keep the original LMP due date.
- If the ultrasound date differs by more than 7 days, your official medical due date will be permanently changed to match the ultrasound measurements.
IVF Due Date Calculations
If you conceived via In Vitro Fertilization (IVF), Naegele's rule is thrown out entirely. Because the exact age of the embryo and the date of transfer are perfectly known, the calculation is absolute.
For a Day-5 embryo transfer, the due date is exactly 261 days from the transfer date. For a Day-3 transfer, it is 263 days.
Pregnancy Milestones & Term Definitions
The Danger Zone (Preterm)
The Safe Zone (Full Term)
The Waiting Game (Postterm)
Breaking Down the Trimesters
Pregnancy is divided into three trimesters. Each phase brings distinct physical changes for the mother and critical developmental milestones for the fetus.
First Trimester (Weeks 1 to 12)
This is the foundation phase. By the end of week 12, the fetus has formed all of its major organs and body systems. Because of the rapid hormonal surge (specifically hCG and progesterone), this is when mothers experience the most intense fatigue, nausea ("morning sickness"), and breast tenderness.
Second Trimester (Weeks 13 to 26)
Often referred to as the "honeymoon phase" of pregnancy. The placenta takes over hormone production, which usually causes nausea to fade and energy to return. The risk of miscarriage drops dramatically. During this phase, you will feel the baby's first movements (quickening) and the anatomy ultrasound will confirm fetal health.
Third Trimester (Weeks 27 to End)
The growth and maturation phase. The fetus rapidly gains weight, putting intense physical pressure on the mother's internal organs. Discomforts like heartburn, shortness of breath, and back pain become common. The focus shifts to preparing for labor and delivery.
Frequently Asked Questions
Can I plan to have my baby on a specific date?
Why does my pregnancy app say I'm 4 weeks pregnant when I just missed my period?
Is it safe to fly during my third trimester?
Sources & Citations
- ACOG Committee Opinion No. 700: Methods for Estimating the Due Date— American College of Obstetricians and Gynecologists
- Naegele's Rule and Pregnancy Dating— National Institutes of Health (NIH)
- Fetal Growth and Ultrasound Guidelines— Society of Obstetricians and Gynaecologists of Canada (SOGC)
What to Calculate Next?
Once the baby arrives, the expenses begin. Start planning your family budget now by running the numbers on the total cost of diapers over the next three years.
Calculate Total Diaper CostsDr. Marcus Thorne
Health & Vitality Consultant | MD, PhD
“Dr. Thorne is a board-certified physician focused on preventative medicine. He ensures our health and fitness calculators align with the latest clinical guidelines and nutritional science.”