What is a Pregnancy Due Date?
A pregnancy due date (also called estimated date of delivery or EDD) is the predicted date when a baby will be born, calculated as approximately 40 weeks (280 days) from the first day of the last menstrual period (LMP). This date helps healthcare providers monitor fetal development, schedule prenatal tests, and plan for delivery.
It's important to understand that only about 5% of babies are born exactly on their due date. Most babies (about 80%) are born within two weeks before or after the estimated due date. A full-term pregnancy is considered anywhere from 37 to 42 weeks.
Why Calculate Your Due Date:
- Prenatal Care Planning: Schedule important tests and ultrasounds at the right times
- Fetal Development Tracking: Monitor baby's growth milestones week by week
- Health Monitoring: Identify potential complications like preterm or post-term pregnancy
- Birth Planning: Prepare for maternity leave, hospital arrangements, and delivery preferences
- Medical Decision Making: Determine appropriate timing for interventions if needed
- Personal Preparation: Plan nursery setup, baby supplies, and family support
How to Calculate Due Date
The most common method for calculating due date is Naegele's Rule, developed in the 1800s and still used worldwide today. This rule assumes a regular 28-day menstrual cycle with ovulation occurring on day 14.
Naegele's Rule Formula:
Due Date = LMP + 280 days (40 weeks)
Or: LMP + 1 year - 3 months + 7 days
Example: If LMP is January 1, 2025:
January 1 + 280 days = October 8, 2025
Or: January 1 + 1 year = January 1, 2026
Minus 3 months = October 1, 2025
Plus 7 days = October 8, 2025
Alternative Calculation Methods:
Conception Date Method
If you know the exact conception date (from fertility treatment or known intercourse date), add 266 days (38 weeks).
Formula: Conception Date + 266 days = Due Date
Ultrasound Dating
First-trimester ultrasound (before 14 weeks) is the most accurate method, with ±5-7 days accuracy. Measures crown-rump length (CRL) to estimate gestational age.
Accuracy: Most accurate at 8-13 weeks. Later ultrasounds are less precise due to growth variations.
IVF/Fertility Treatment
For IVF pregnancies, due date is calculated from embryo transfer date plus embryo age (3-day or 5-day embryo).
Formula: Transfer date + 266 days - embryo age at transfer
Important Note:
Healthcare providers may adjust your due date based on first-trimester ultrasound if it differs from LMP calculation by more than 5-7 days. The ultrasound date is typically considered more accurate.
Factors Affecting Due Date Accuracy
While due date calculations provide a helpful estimate, several factors can affect accuracy and actual delivery timing:
Irregular Menstrual Cycles
Naegele's Rule assumes a 28-day cycle with day-14 ovulation. Women with longer, shorter, or irregular cycles may ovulate earlier or later, affecting conception date and true due date. Cycles ranging from 21-35 days are normal, but this variability makes LMP-based dating less accurate.
First Pregnancy vs. Subsequent Pregnancies
First-time mothers (primiparous) tend to deliver slightly later, averaging 40 weeks + 5 days. Women who have given birth before (multiparous) typically deliver around 40 weeks + 3 days. However, individual variation is significant.
Maternal Age and Ethnicity
Advanced maternal age (35+) is associated with slightly higher rates of post-term pregnancy. Different ethnic groups show small variations in average gestational length, though these differences are typically only 2-4 days and have minimal clinical significance.
Fetal Factors
Baby's sex affects timing slightly (males average 1 day longer). Multiples (twins, triplets) almost always deliver early - twins average 35-37 weeks, triplets 32-34 weeks. Fetal size and genetic factors also play roles.
Medical Conditions
Conditions like gestational diabetes, preeclampsia, or placental issues may necessitate early delivery. Conversely, some conditions may prolong pregnancy. Medical interventions (induction, C-section) override natural timing.
⚕️ Medical Disclaimer
For Informational Purposes Only
This calculator provides estimates only and is not a substitute for professional prenatal care.
This pregnancy due date calculator provides estimated dates for educational purposes only. It should not replace professional medical advice, prenatal care, or clinical dating.
Frequently Asked Questions
How accurate is the pregnancy due date calculator?
LMP-based calculators are accurate to within ±1-2 weeks for most women with regular cycles. First-trimester ultrasound (8-13 weeks) is more accurate at ±5-7 days. Only about 5% of babies are born on their exact due date, but 80% arrive within 2 weeks either side of it.
What if I don't know my last menstrual period date?
If you don't remember your LMP, your healthcare provider will use first-trimester ultrasound to date your pregnancy. This measures the embryo's crown-rump length (CRL), which is very accurate for determining gestational age. Irregular cycles also require ultrasound dating.
Why is pregnancy 40 weeks but 9 months?
Pregnancy is counted from the first day of your last period (not conception), so it includes the 2 weeks before ovulation. 40 weeks equals 280 days or approximately 9 months and 1 week. However, months vary in length (28-31 days), while weeks are consistent, making weeks a more precise measurement.
Can my due date change?
Yes. Healthcare providers may adjust your due date if a first-trimester ultrasound differs from LMP calculations by more than 5-7 days. The ultrasound date is usually considered more accurate. After 20 weeks, due dates are rarely changed unless there was an error in initial dating.