Due Date Calculator
About the Due Date Calculator
Planning around a pregnancy is easier when you can see the dates laid out clearly, and ProCalc.ai’s Due Date Calculator helps you do exactly that. You use the Due Date Calculator to estimate your due date and map out your pregnancy timeline, including trimester start and end dates, so you can keep appointments, work plans, and personal milestones in sync. It’s commonly used by people who’ve just had a positive test and want a realistic timeline to share with their OB-GYN or midwife at the first visit. For example, if you’re scheduling your first prenatal appointment and trying to figure out when the 12‑week scan might fall, this gives you a quick, consistent estimate to work from. You enter your last menstrual period (or your conception date if you know it), and you get an estimated due date plus a week-by-week view and trimester breakdown to help you understand where you are right now and what’s coming next.
How is the due date calculated?
The most common method (Naegele's Rule) adds 280 days (40 weeks) to the first day of your last menstrual period. If calculating from conception, it adds 266 days (38 weeks).
What Is the Due Date Calculator?
The Due Date Calculator estimates a pregnancy's estimated due date (EDD) from either the first day of the last menstrual period (LMP), the conception date, an IVF transfer date, or an ultrasound measurement. It also returns current gestational age in weeks and days and the start dates of each trimester.
How to Use This Calculator
Pick your input method. The most common is LMP: enter the first day of the last menstrual period and the calculator applies Naegele's rule (LMP + 280 days) to project the due date. For IVF, enter the transfer date and embryo age (day 3 or day 5) for higher precision. For ultrasound dating, enter the scan date and the measured gestational age.
Common Use Cases
- First-trimester planning: Knowing the due date sets the schedule for the 12-week NIPT screening, the 20-week anatomy scan, and the 28-week glucose tolerance test.
- Maternity leave timing: Most U.S. employers count 12 weeks of FMLA leave from the birth date — knowing the EDD helps coordinate handoff with your team.
- IVF cycle tracking: Day 5 frozen embryo transfers add 261 days to the transfer date for the EDD; day 3 transfers add 263 days.
- Conception date back-calculation: Helpful for paternity questions or when LMP is uncertain.
Understanding the Results
Only about 5% of babies arrive on the actual due date — the EDD is a 40-week midpoint, with normal-term delivery anywhere from 37 to 42 weeks. First-trimester ultrasound dating (before 14 weeks) is the most accurate reference and is what your OB will use for the official chart EDD if it differs from LMP by more than 7 days. Gestational age is always counted from LMP, not from conception, which is why pregnancy is technically "10 lunar months" or 280 days.
Industry Standards and Tips
ACOG (the American College of Obstetricians and Gynecologists) recommends using the earliest accurate ultrasound for due date assignment when available. The 280-day Naegele's rule assumes a 28-day cycle and ovulation on day 14 — adjust by your typical cycle length if it varies meaningfully. Babies born before 37 weeks are preterm; after 42 weeks are post-term and usually induced.
For age-related fertility planning, see other Health calculators on ProCalc.ai.
Due Date Calculator — Frequently Asked Questions(8)
Common questions about due date.
Last updated Apr 2026
How Pregnancy Due Dates Are Calculated
A due date (estimated date of delivery, or EDD) is calculated by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). This is known as Naegele's rule, developed in the early 19th century and still the standard method used by obstetricians worldwide. The 40-week calculation assumes a 28-day cycle with ovulation occurring on day 14.
Using the Calculator
Enter the first day of your last menstrual period and your average cycle length. The calculator shows your estimated due date, current gestational age in weeks and days, trimester, and key milestone dates throughout pregnancy. If your cycle is longer or shorter than 28 days, the calculator adjusts the due date accordingly.
Trimester Milestones
First trimester (weeks 1-12): Major organ formation occurs. First prenatal visit typically at 8-10 weeks. First ultrasound confirms viability and may adjust the due date. Second trimester (weeks 13-27): Anatomy scan at 18-22 weeks. Fetal movement felt around 16-25 weeks. Glucose screening at 24-28 weeks. Third trimester (weeks 28-40): Growth monitoring, Group B strep test at 36 weeks, weekly visits from 36 weeks onward.
Due Date Accuracy
Only about 5% of babies are born on their exact due date. The due date represents the midpoint of a normal delivery range — full term is defined as 39 weeks 0 days through 40 weeks 6 days by the American College of Obstetricians and Gynecologists. Deliveries between 37 and 42 weeks are considered within the normal range. First pregnancies tend to run slightly longer on average.
Ultrasound Dating vs LMP Dating
A first-trimester ultrasound measuring crown-rump length is the most accurate method for determining gestational age — accurate to within ±5-7 days. If the ultrasound date differs from the LMP-calculated date by more than 7 days, the ultrasound date is generally preferred. Later ultrasounds are less precise for dating (±2-3 weeks in the third trimester).
Cycle Length Adjustments
Naegele's rule assumes a 28-day cycle. For longer cycles, ovulation occurs later, pushing the due date forward. A woman with a 35-day cycle likely ovulated around day 21 instead of day 14 — adding 7 days to the standard due date. The calculator makes this adjustment automatically when you enter your average cycle length.
Authoritative Sources
This calculator uses formulas and reference data drawn from the following sources:
- CDC — Physical Activity - NIH — National Institute of Diabetes - NIH — National Heart, Lung, and Blood Institute
Due Date Formula & Method
This due date calculator uses standard health formulas to compute results. Enter your values and the formula is applied automatically — all math is handled for you. The calculation follows industry-standard methodology.
Due Date Sources & References
Explore More Calculators
Content reviewed by the ProCalc.ai editorial team · About our standards
🔀 You Might Also Use
Mortgage Calculator
Free mortgage calculator with payment breakdown, amortization schedule, extra payment scenarios, and 15 vs 30 year comparison.
FINANCEBMI Calculator
Free BMI Calculator — Calculate BMI. Check if weight is healthy based on height. AI-powered health tool.
HEALTHAge Calculator
Free Age Calculator — Calculate your age in years, months, days. Find day of week you were born. Instant results.
MATHTip Calculator
Calculate your tip and split the bill between any number of people. Choose any percentage — works for restaurants, delivery, hair salons, and services.
FINANCEExplore More Health Tools
Pace Calculator
Free Pace Calculator — Calculate your running or walking pace per mile or kilometer. Enter distance and time to get pace, speed, and projected ...
Ideal Body Weight Calculator
Free Ideal Body Weight Calculator — Calculate your ideal body weight using the Devine formula based on height and gender.
TDEE Calculator
Calculate your Total Daily Energy Expenditure, macro split, and body composition. Free TDEE calculator with BMR formulas, meal planning, and weight.
BMR Calculator
Free BMR calculator using Mifflin-St Jeor and Harris-Benedict formulas. Enter age, weight, and height to find calories burned at rest — instant results.
BMI Calculator
Free BMI Calculator — Calculate BMI. Check if weight is healthy based on height. AI-powered health tool.
