Understanding Your Estimated Due Date
Your estimated due date (EDD) is not a deadline — it is the midpoint of a normal delivery window. Full-term pregnancy is defined as 37-42 weeks, and only about 4% of babies arrive on their calculated due date. Understanding this removes unnecessary anxiety and helps you plan realistically for a window of arrival rather than a specific date.
The standard calculation (Naegele's rule) adds 280 days to the first day of your last menstrual period. This assumes a 28-day cycle with ovulation on day 14. For women with longer or shorter cycles, adjusting by the difference from 28 days improves accuracy. A 35-day cycle means ovulation around day 21 — pushing the EDD approximately 7 days later than the standard calculation would suggest.
Early ultrasound (ideally between 8-13 weeks) is considered the gold standard for dating. It measures crown-rump length and compares to population norms, providing an accurate gestational age. If ultrasound dating differs from the LMP-based calculation by more than 5-10 days, the ultrasound date typically takes precedence as the working EDD.
Calculate your due date
Enter your last menstrual period or conception date to get your estimated due date, current week of pregnancy, and full milestone calendar.
Calculate My Due DateKey Pregnancy Milestones to Know
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Week 6-8: First prenatal visit
Initial appointment typically includes pregnancy confirmation, first ultrasound (may detect heartbeat around week 6), blood tests, health history, and establishing care. Due date may be confirmed or adjusted at this visit based on ultrasound measurements.
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Week 10-13: First trimester screening
Combined screening for chromosomal abnormalities — nuchal translucency ultrasound plus blood tests. NIPT (non-invasive prenatal testing) can also be done from week 10. CVS (chorionic villus sampling) is available for diagnostic testing if indicated. These are the primary windows for early genetic screening.
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Week 18-22: Anatomy scan
Detailed ultrasound examining fetal anatomy — brain, heart, spine, kidneys, and limbs. Often the appointment where parents learn the sex if desired. Placenta location is assessed; placenta previa is identified if present. This is typically the most comprehensive structural evaluation of the pregnancy.
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Week 24-28: Glucose screening
Gestational diabetes screening (glucose challenge test). If elevated, a 3-hour glucose tolerance test follows. Rhogam injection for Rh-negative mothers at week 28. Third-trimester prenatal visits become more frequent — typically every 2 weeks from week 28-36, then weekly.
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Week 37: Full term
Baby is considered full term — lungs, brain, and organs are mature. Most planned inductions and elective cesareans are not scheduled before this point. From week 37, weekly prenatal visits are standard until delivery. Post-dates monitoring typically begins at week 40-41.
How Gestational Age Is Counted
Gestational age is counted from the first day of the last menstrual period — not from conception. This means at the moment of conception (approximately 2 weeks after LMP in a 28-day cycle), you are already considered to be 2 weeks pregnant by clinical convention. At a positive pregnancy test (typically around week 4), you are already 4 weeks along in gestational counting.
This convention can be initially confusing but is universal in clinical practice because the LMP date is reliably known whereas conception date is rarely known precisely. All ultrasound measurements, screening windows, and care protocols are referenced to gestational weeks counted from LMP.
How to Use This Calculator
The due date calculator accepts either your last menstrual period date or a known conception date, along with your average cycle length. It outputs your estimated due date, current gestational week, trimester, and a full milestone calendar showing key prenatal screening windows and appointments.
If you have already had an early ultrasound with a different EDD than the LMP calculation, use the ultrasound-based date for subsequent planning — it is more accurate. The calculator can also be run forward from a known early ultrasound date to project the full pregnancy timeline.
Frequently Asked Questions
Can my due date change after it is set?
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Yes. Due dates are frequently revised after early ultrasound. If measurements differ significantly from the LMP-based calculation, the ultrasound date becomes the working EDD. Due dates can also be informally updated by later ultrasounds, though early ultrasound (8-13 weeks) remains the most accurate dating method and is not typically changed by later scans.
What happens if I go past my due date?
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Post-dates pregnancy (after 40 weeks) is common. Between 40-41 weeks, monitoring typically increases with non-stress tests (NST) and biophysical profiles. At 41-42 weeks, induction is often recommended as placental function begins to decline. Delivery after 42 weeks carries increased risk for the baby and is generally managed medically.
Is it okay to deliver before my due date?
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Delivery before 37 weeks is preterm and carries gestational-age-dependent health risks. Between 37-38 weeks (early term), babies have higher rates of respiratory complications than 39-40 week births. Medically indicated early delivery for conditions like preeclampsia or IUGR is sometimes necessary despite these risks. Elective induction before 39 weeks without medical indication is generally avoided.
How accurate is Naegele's rule?
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Naegele's rule is accurate within its assumptions — 28-day cycles with ovulation at day 14. For women with irregular cycles, very long cycles, or PCOS, the LMP-based calculation may be off by 1-2 weeks. This is why early ultrasound is valuable: it provides a gestational age estimate independent of cycle length assumptions and is accurate to within plus or minus 5-7 days when performed before 13 weeks.
What is the difference between gestational age and fetal age?
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Gestational age counts from LMP. Fetal age (sometimes called embryonic age) counts from conception — typically about 2 weeks after LMP in a standard cycle. Clinical medicine universally uses gestational age. Fetal age is approximately 2 weeks less than gestational age. When your OB says your baby is 20 weeks, they mean 20 weeks gestational age, which corresponds to approximately 18 weeks of fetal development.
What prenatal vitamins are important and when should I start?
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Folic acid (400-800 mcg daily) is the most critical prenatal supplement and should ideally be started before conception — neural tube development occurs in weeks 3-4, often before a pregnancy is confirmed. Other key nutrients in prenatal vitamins include iron, calcium, vitamin D, iodine, and DHA. Starting a prenatal vitamin as soon as you are planning to conceive or as soon as you know you are pregnant is universally recommended.
Calculate your pregnancy timeline
Get your estimated due date, current gestational week, trimester, and a full milestone calendar from now through delivery.
Calculate My Due Date