Your Estimated Due Date
Day of Week
Weeks
Days left
Trimester
Delivery Window
How Is Your Due Date Calculated?
Your estimated due date (EDD) is calculated using Naegele's Rule, a formula developed in the early 1800s that adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This method assumes a 28-day cycle with ovulation occurring on day 14, though modern calculators like ours can adjust for different cycle lengths to improve accuracy. Understanding how your due date is determined helps you better interpret prenatal care decisions and prepare for your baby's arrival.
Why Might Your Due Date Change After an Ultrasound?
Early ultrasounds (typically between 7-12 weeks) measure your baby's crown-rump length to estimate gestational age, often more accurately than LMP-based calculations. If the ultrasound date differs from your LMP date by more than a week, your healthcare provider may adjust your official due date. This is completely normal and doesn't mean anything is wrong—it simply reflects that not everyone ovulates on the textbook day 14. Having an accurate due date is important for monitoring fetal growth and making decisions about induction if you go past your date. Track your current week with our pregnancy week calculator.
What Does "Full Term" Really Mean?
The definition of full term has evolved based on research showing that babies born at different weeks have different outcomes. Current medical guidelines classify term pregnancies as: Early Term (37 weeks 0 days to 38 weeks 6 days), Full Term (39 weeks 0 days to 40 weeks 6 days), Late Term (41 weeks 0 days to 41 weeks 6 days), and Post-Term (42 weeks and beyond). The ideal window for delivery is the full-term period between 39 and 41 weeks, when babies have the best outcomes. Unless there's a medical reason, most providers recommend waiting until at least 39 weeks for scheduled deliveries.
What Happens If You Go Past Your Due Date?
Going past your due date is common—only about 5% of babies arrive on their exact due date, and many first-time mothers deliver around 41 weeks. If you reach 41 weeks, your provider will likely recommend increased monitoring, including non-stress tests and amniotic fluid checks. Most providers discuss induction between 41 and 42 weeks because placental function can decline and complications increase after this point. Every situation is unique, so discuss your specific circumstances and preferences with your healthcare team. If you're wondering about the early signs, our contraction timer can help when labor begins.
How Accurate Is a Due Date Calculator?
Due date calculators provide an estimate, not a guarantee. Even with perfect information, predicting exactly when labor will begin is impossible. Studies show that among naturally conceived pregnancies with accurate dating, about 70% of women deliver within 10 days of their due date, and 90% deliver within two weeks either way. Factors like whether this is your first pregnancy, your age, and the baby's position can all influence timing. Think of your due date as the center of a window rather than a specific target. For those who used fertility treatments, our IVF due date calculator provides more precise dating based on your transfer information.
Frequently Asked Questions
Can I calculate my due date without knowing my last period?
Yes, but you'll need either a known conception date, IVF transfer date, or an ultrasound measurement. If you have irregular periods or weren't tracking, an early ultrasound is the most reliable way to determine your due date. The earlier the ultrasound, the more accurate the dating.
Why is pregnancy counted as 40 weeks when it's only 9 months?
Pregnancy is dated from the first day of your last period, which is about 2 weeks before conception. So while you're "pregnant" for 40 weeks by this counting method, your baby is actually developing for about 38 weeks. This dating system exists because LMP is easier to identify than the actual conception date.
Does my due date change if I have a longer or shorter cycle?
It should. If you have a 35-day cycle, you likely ovulated around day 21, not day 14, so your due date would be about a week later than standard LMP calculations suggest. Some calculators adjust for this, or your provider may adjust based on ultrasound findings.
How do I know if I should be induced?
Induction decisions are based on many factors: how far past your due date you are, your health conditions, the baby's wellbeing on monitoring, and your preferences. Most providers recommend discussing induction by 41 weeks, with strong recommendations for induction by 42 weeks to reduce risks.
Is it true that first babies come late?
Research does show that first-time mothers are more likely to deliver after their due date compared to women who've had babies before. However, this isn't universal—plenty of first babies arrive early or right on time. Use the due date as a guide, not a guarantee.
Can stress or activity change my due date?
Your due date itself won't change (it's calculated from fixed points like LMP or conception), but stress, activity, and other factors might influence when labor begins. However, evidence doesn't support that moderate activity or normal stress levels significantly affect delivery timing.