Free Due Date Calculator
Free Pregnancy Due Date Calculator - When Is My Baby Due?
Normal range: 21–35 days. Average: 28 days.
Enter a date to calculate your estimated due date 🍼
How This Calculator Works
Naegele's Rule
This calculator uses Naegele's rule, the standard method used by obstetricians since 1812. The formula adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. For cycles shorter or longer than 28 days, the calculator adjusts by adding or subtracting the difference. For example, a 30-day cycle adds 2 extra days to the due date. While simple, Naegele's rule is remarkably accurate for women with regular cycles. Your healthcare provider may adjust your due date after a first-trimester ultrasound, which can date pregnancy within ±5-7 days.
Why Only 5% Arrive On Time
Despite its name, a "due date" is really a "due estimate." Only about 4-5% of babies are born on their exact due date, according to the American College of Obstetricians and Gynecologists (ACOG). About 80% of babies arrive between 37 and 42 weeks. First-time mothers tend to deliver slightly later — an average of 41 weeks and 1 day. The due date is better understood as the middle of a "due window" spanning roughly 37-42 weeks. Factors affecting timing include genetics, maternal age, and whether it's a first pregnancy. Use our workday calculator to plan maternity leave timing.
Trimester Breakdown
First trimester (weeks 1-12): Major organ development. Morning sickness is common. Miscarriage risk is highest. First heartbeat at week 6. Second trimester (weeks 13-26): Often called the "honeymoon phase" — energy returns, morning sickness fades. Baby's gender visible around week 16. Anatomy scan at week 20. Third trimester (weeks 27-40): Rapid growth. Baby can survive outside the womb from week 24 (with intensive care). Full term begins at week 37. Braxton Hicks contractions are common. The baby "drops" (engages) in late pregnancy.
IVF Due Date Calculation
IVF due dates are the most accurate because the exact conception date is known. For a 5-day blastocyst transfer (the most common), we subtract 5 days from the transfer date to estimate conception, then add 266 days (38 weeks from conception). For a 3-day transfer, subtract 3 days instead. IVF due dates typically don't need adjustment because there's no guesswork about ovulation timing. Your fertility specialist will confirm your due date at your first ultrasound, usually around 6-8 weeks gestational age.
Key Pregnancy Milestones
Major milestones during pregnancy: Week 6: First heartbeat (120-160 bpm). Week 8: Dating ultrasound for accurate due date. Week 12: NT scan, end of highest miscarriage risk. Week 16: Gender may be visible. Week 20: Anatomy scan checks all major organs. Week 24: Viability — baby could survive with NICU care. Week 28: Third trimester, more frequent doctor visits. Week 37: Full term. Week 40: Due date. Week 42: Post-term, induction usually recommended.
Factors Affecting Due Date
Several factors can shift your actual delivery date from the estimate: Irregular cycles: Longer cycles mean later ovulation and a later due date. First pregnancy: First-time mothers average 41+ weeks. Maternal age: Women over 35 are more likely to deliver early. Multiple pregnancies: Twins average 36 weeks, triplets 32 weeks. Genetics: If your mother delivered late, you might too. Lifestyle: Stress, nutrition, and exercise all play roles. Always discuss your individual factors with your OB/GYN or midwife. Check our BMI calculator for pregnancy health considerations.
Frequently Asked Questions
How is a due date calculated?+
Due dates are calculated using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). For a standard 28-day cycle, this assumes ovulation on day 14 and fertilization shortly after. If your cycle is longer or shorter, adjust by adding or subtracting the difference from 28 days. Conception date calculations add 266 days (38 weeks). First-trimester ultrasounds can refine the estimate to within ±5-7 days.
What percentage of babies are born on their due date?+
Only about 4-5% of babies are born on their exact due date. About 60% are born within a week of their due date, and 80% arrive between 37 and 42 weeks. The average first-time mother delivers at 41 weeks and 1 day. Think of the due date as the center of a 5-week window (37-42 weeks) rather than a precise prediction.
How accurate is a due date calculator?+
LMP-based calculations are accurate within about 2 weeks for women with regular cycles. First-trimester ultrasound dating (the gold standard) narrows this to ±5-7 days. IVF due dates are the most accurate since conception timing is precisely known. Irregular cycles, uncertain LMP dates, and individual variation all reduce accuracy. Your healthcare provider will give you the most reliable estimate after considering all available data.
When does each trimester start and end?+
First trimester: weeks 1-12 (from LMP through the end of week 12). Second trimester: weeks 13-26 (baby grows rapidly, most comfortable period for many women). Third trimester: weeks 27-40 (preparing for birth). Full term officially begins at week 37. Most providers consider weeks 39-40 as ideal for delivery, as the baby's lungs and brain are fully developed by this point.
Deep Dive: How Pregnancy Dating Works
The Naegele's Rule, developed by German obstetrician Franz Karl Naegele around 1850, remains the foundation of due date calculation: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This 280-day figure assumes a 28-day cycle with ovulation at day 14, making fertilization approximately 14 days after LMP — meaning gestational age includes two weeks before conception occurred. This counterintuitive convention persists because women reliably know their LMP but rarely know the exact day of conception. A 40-week gestation is actually only about 38 weeks of actual fetal development.
First-trimester ultrasound (typically 8-13 weeks) provides the most accurate gestational age estimate, measuring crown-rump length (CRL). At this stage, fetal growth is predictable enough that CRL measurement has an accuracy of ±5-7 days. When ultrasound dating differs from LMP dating by more than 5-7 days in the first trimester (or 10-14 days in the second), most guidelines recommend revising the estimated due date to match ultrasound findings. A 2015 systematic review in Ultrasound in Obstetrics & Gynecology confirmed first-trimester ultrasound as the gold standard for gestational age assessment.
Only about 5% of babies are born on their exact due date. Normal term pregnancy spans 37-42 weeks — the 'due date' is better understood as the center of a 5-week window. Among spontaneous (non-induced) deliveries, a 2013 BJOG study found the median delivery for first-time mothers was 40 weeks 5 days; for subsequent pregnancies, 40 weeks 3 days. Post-term pregnancy (beyond 42 weeks) carries increasing risk of fetal compromise and placental insufficiency, which is why most providers offer or recommend induction at 41-42 weeks.
Preterm birth — delivery before 37 weeks — affects approximately 10% of births globally and is the leading cause of neonatal death. Risk factors include multiple gestation (twins/triplets), prior preterm birth, cervical insufficiency, uterine abnormalities, infection, and stress. Progesterone supplementation in women with short cervical length has been shown to reduce preterm birth risk. Antenatal corticosteroids given 24-34 weeks accelerate fetal lung maturity and dramatically reduce complications if preterm birth occurs. Understanding gestational milestones — viability at 22-24 weeks, improved outcomes after 28 weeks, near-normal outcomes after 34 weeks — contextualizes the stakes of accurate dating.