Due Date Calculator

Your due date represents the estimated day your baby will arrive, though only about 5% of babies are born on this exact date. Understanding how due dates are calculated, what influences their accuracy, and the normal delivery window helps set realistic expectations for when you'll meet your baby. This knowledge reduces anxiety about "going overdue" and helps you prepare appropriately for labor and delivery.

Due Date Range vs. Specific Date

Full-term pregnancy spans from 37 to 42 weeks, representing a 5-week range during which delivery is considered normal and healthy. Only about 70% of babies arrive within the week before or after their due date, while 30% arrive outside this narrow window but still within the healthy full-term range.

Approximately 10% of babies arrive during weeks 37-38 (early term), 70% during weeks 39-40 (full term), and 15% during weeks 41-42 (late term). Only about 5% of pregnancies reach 42 weeks, at which point induction is typically recommended due to increased risks.

First-time mothers tend to deliver later than experienced mothers, with first babies averaging 40 weeks and 5 days. Women who've given birth before average closer to 40 weeks and 3 days. This difference likely reflects more efficient cervical ripening and stronger contractions in subsequent pregnancies.

Maternal age influences delivery timing, with teenage mothers more likely to deliver preterm and mothers over 35 slightly more likely to go past their due dates. Maternal health conditions, particularly gestational diabetes and preeclampsia, often necessitate delivery before the due date.

Multiple gestations rarely reach 40 weeks. Twin pregnancies average 36 weeks, triplets about 33 weeks, and higher-order multiples even earlier. The uterus reaches capacity sooner with multiple babies, often triggering labor before full term.

When Due Dates Get Adjusted

Providers may adjust due dates based on early ultrasound findings that differ significantly from LMP-based dates. If your LMP suggests 10 weeks but ultrasound measurements indicate 12 weeks, your due date would be adjusted to match the ultrasound, making you 2 weeks more pregnant than you thought.

These adjustments occur most reliably when based on first trimester ultrasounds. A CRL measurement indicating 9 weeks when LMP suggests 11 weeks would result in adjusting the due date back by 2 weeks, assuming the early ultrasound is more accurate than LMP recall.

Due dates shouldn't be adjusted based on third trimester ultrasounds unless there's clear evidence of dating error. A baby measuring large or small in the third trimester usually reflects growth patterns rather than dating mistakes. Changing the due date based on late ultrasounds can lead to inappropriate intervention.

IVF pregnancies use exact conception dates for due date calculation. The due date is calculated as 266 days (38 weeks) from the date of embryo retrieval and fertilization, or for frozen embryo transfers, 266 days minus the embryo's age at freezing. This provides extremely accurate dating.

Assisted reproductive technology also affects calculations based on embryo transfer timing. A 5-day-old embryo (blastocyst) transferred on May 1 has a due date calculated as if conception occurred on April 27, with 266 days added to that backdated conception date.

Naegele's Rule Explained

Naegele's rule, developed in the 1800s, remains the standard method for calculating estimated due dates. The formula adds 280 days (40 weeks) to the first day of your last menstrual period, or more simply: add 7 days to your LMP date, subtract 3 months, and add 1 year.

If your last period began on October 10, 2024, add 7 days to get October 17, subtract 3 months to July 17, then add 1 year for a due date of July 17, 2025. This calculation assumes a 28-day menstrual cycle with ovulation occurring on day 14.

The rule's 280-day timeline accounts for the dating convention that starts pregnancy from LMP rather than conception. Since conception typically occurs about 14 days after LMP (at ovulation), the actual time from conception to delivery is approximately 266 days or 38 weeks. The 40-week pregnancy includes about 2 weeks before conception even occurred.

Naegele's rule works reasonably well for women with regular 28-day cycles but becomes less accurate for those with longer, shorter, or irregular cycles. Women with 35-day cycles ovulate later, around day 21, meaning their actual due date would be about a week later than Naegele's rule predicts. Conversely, women with 21-day cycles ovulate earlier, around day 7, shifting their due date earlier.

Calculation from Last Menstrual Period

LMP-based dating provides a standardized reference point that all healthcare providers use consistently. Even if you know your exact conception date, providers typically still use LMP for official gestational age because it provides more reliable population-level statistics.

Accurate LMP dating requires remembering the first day of your last normal menstrual period before pregnancy. Some women mistakenly use the last day of their period or the date of a light spotting episode that might have actually been implantation bleeding. Using the wrong start date shifts your due date by days or even weeks.

Women with irregular periods or those who were using hormonal birth control recently may not have reliable LMP dates. If your cycles varied from 21 to 45 days, LMP-based calculation produces unreliable estimates. In these cases, early ultrasound dating provides more accurate due dates.

Calculating due dates becomes complicated if you conceived while breastfeeding, immediately after stopping birth control, or during perimenopause when cycles are irregular. These situations often require relying more heavily on ultrasound measurements than LMP dating.

Online due date calculators streamline the process by automatically adding 280 days to your LMP date, but the underlying calculation remains Naegele's rule. These tools also typically show your current gestational age and countdown to your due date, helpful for tracking pregnancy progression.

Preparing for the Delivery Window

Rather than fixating on your specific due date, prepare for labor anytime during weeks 37-42. Have hospital bags packed by week 36, install car seats, and complete work transitions. Babies don't consult calendars before arriving.

Understand that going past your due date is completely normal. More than 40% of first-time mothers deliver after their due date. Unless medical complications arise, most providers allow pregnancies to continue to 41-42 weeks before recommending induction.

At 41 weeks, many providers offer membrane stripping to encourage labor naturally. This procedure separates the amniotic sac from the lower uterus, releasing prostaglandins that might trigger labor within 24-48 hours. It's uncomfortable but avoids medical induction.

Induction is typically recommended at 41-42 weeks because placental function declines after 42 weeks, increasing risks of stillbirth, meconium aspiration, and fetal distress. While the absolute risks remain low, induction at 41-42 weeks reduces these risks compared to expectant management.

Non-stress tests and biophysical profiles monitor fetal wellbeing in late-term pregnancies. These tests assess fetal heart rate patterns, movement, breathing practice, and amniotic fluid levels. Reassuring results indicate the baby tolerates continued pregnancy well.

Ultrasound Dating Accuracy

First trimester ultrasounds provide the most accurate due date predictions, typically within 3-5 days of actual delivery date. During weeks 8-13, embryonic development follows highly predictable patterns, making measurements extremely reliable for dating.

Crown-rump length (CRL) measurement from the top of the head to the bottom of the spine correlates precisely with gestational age in early pregnancy. A CRL of 45mm corresponds almost exactly to 11 weeks gestation regardless of maternal factors or fetal genetics that later influence size.

Second trimester ultrasounds remain fairly accurate for dating, within about 7-10 days, but become less precise than first trimester scans. The anatomy scan at 20 weeks uses multiple measurements—head circumference, abdominal circumference, femur length—to estimate gestational age.

Third trimester ultrasounds provide poor dating accuracy, with margins of error reaching 2-3 weeks. By this point, fetal size varies substantially based on genetics, nutrition, and individual growth patterns. A large baby might measure 3 weeks ahead while actually being right on schedule, or a small baby might measure behind despite being perfectly healthy.

If ultrasound dating differs from LMP dating by more than 5-7 days in the first trimester or 10-14 days in the second trimester, providers typically adjust the due date to match ultrasound findings. Early ultrasounds are considered more reliable than potentially faulty memory of LMP dates.

Factors Affecting Accuracy

Cycle length dramatically impacts due date accuracy when using LMP-based calculation. Women with 35-day cycles ovulate around day 21, not day 14, meaning they're a week less pregnant than LMP dating suggests. Their actual due date is approximately one week later than Naegele's rule predicts.

Irregular cycles create the greatest dating uncertainty. If your cycles varied from 25 to 40 days, you could have ovulated anywhere from day 11 to day 26, making LMP-based dating essentially guesswork. Early ultrasound becomes crucial for establishing accurate due dates in these cases.

Conception timing affects actual gestation length even with accurate dating. While average time from conception to delivery is 266 days, normal variation spans 255-275 days. Some babies are simply ready earlier or later than average, independent of dating accuracy.

Ethnicity shows slight variations in average pregnancy length. Studies suggest Asian women average slightly shorter gestations (39.5 weeks) while African American women average slightly longer (40.3 weeks), though individual variation far exceeds these population-level differences.

Placental health influences delivery timing. An aging placenta might trigger labor earlier, while an exceptionally healthy placenta might support pregnancy well past the due date. Providers monitor placental function in late-term pregnancies through non-stress tests and biophysical profiles.

Managing Expectations and Anxiety

Many women experience anxiety as their due date approaches and passes. Well-meaning comments about "still being pregnant" increase pressure and disappointment. Remember that due dates are estimates, not guarantees or deadlines.

Babies come when they're ready within the safe window of 37-42 weeks. Try not to schedule visitors, work commitments, or partner's time off based on your exact due date. Build flexibility into plans to accommodate normal variation.

Every day past your due date can feel endless when you're uncomfortable and eager to meet your baby. Stay busy with low-key activities, practice relaxation techniques, and try to enjoy these final days before your life changes dramatically.

Communicate with your provider about their post-dates protocols. Some practices schedule induction at 41 weeks while others wait until 42 weeks. Knowing the plan reduces anxiety about what happens if you "go overdue."

Understanding that due dates are educated estimates rather than precise predictions helps you approach the final weeks of pregnancy with patience and realistic expectations, appreciating that your baby will arrive in their own time within the safe delivery window.