Due Date Calculator
Calculate your pregnancy due date by last menstrual period (LMP), conception date, or IVF transfer date. See trimester breakdown, pregnancy progress, and key milestone weeks.
Pregnancy Calculator - Calculate Due Date by LMP or Conception
Calculate your estimated pregnancy due date by pregnancy calculator using three methods: last menstrual period (LMP) using Naegele's Rule, estimated conception date, or IVF embryo transfer date. See your complete pregnancy timeline, including the current week, days remaining, trimester breakdown with start and end dates, and key milestone weeks from heartbeat detection at week 6 through full term at week 37. Pregnancy due date calculation is based on a standard 40-week (280-day) pregnancy measured from the first day of your last menstrual period. Most pregnancies deliver between 38 and 42 weeks—only about 5% of babies are born on their exact estimated due date. Your due date is a target window, not a precise prediction. Always confirm your due date with your healthcare provider through ultrasound, which provides the most accurate dating, especially in the first trimester.
How Pregnancy Due Date is Calculated - Naegele's Rule Explained
Naegele's Rule, developed by German obstetrician Franz Karl Naegele in 1812, remains the standard method for calculating due dates. Formula: Due date equals first day of last menstrual period plus 280 days (40 weeks). Alternative formula: Add 1 year, subtract 3 months, add 7 days to LMP date. Example: LMP January 1, 2025. The due date equals January 1 plus 280 days equals October 8, 2025. The formula assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, due date adjusts accordingly. 35-day cycle: add 7 days to standard due date. 21-day cycle: subtract 7 days. Our calculator automatically adjusts for cycle length between 21-35 days. Important: the first 2 weeks of the 40-week pregnancy are counted before conception even occurs (from LMP, not conception). Actual fetal development is approximately 38 weeks.
Pregnancy Trimesters: What Happens Each Stage
First Trimester (Weeks 1-13): Most critical period for fetal development. All major organs begin forming. Morning sickness peaks weeks 6-10 for most women. Miscarriage risk is highest (10-15% of known pregnancies) but drops significantly after week 12. Heartbeat detectable via ultrasound as early as week 6. First prenatal appointment typically weeks 8-10. First trimester screening (nuchal translucency ultrasound) weeks 11-14. Second trimester (weeks 14-27): The most comfortable trimester for most women. Morning sickness typically resolves. Energy increases. A baby's sex may be visible on ultrasound from week 16. Anatomy scan at week 20—comprehensive ultrasound checking all major structures. Baby begins moving (quickening) is felt by the mother typically weeks 18-22 for first pregnancies and weeks 16-18 for subsequent ones. Third Trimester (Weeks 28-40): Rapid growth. Baby gains approximately 0.5 lb per week. Braxton Hicks contractions begin. "Full term" is defined as 37 weeks (previously 38-42 weeks, the definition changed by ACOG in 2013). Early term: 37-38 weeks. Full term: 39-40 weeks. Late term: 41 weeks. Post-term: 42 weeks or more.
Key Pregnancy Milestones by Week
Week 4: Missed period, positive home pregnancy test likely. hCG levels detectable. Week 6: Heartbeat visible on transvaginal ultrasound. Embryo approximately 5-6mm. Week 8: All major organs have begun forming. Arms and legs visible. Week 10: Embryonic period ends, fetal period begins. All organs present, though immature. Week 12: End of first trimester. Risk of miscarriage drops dramatically. Nuchal translucency screening is done now. Week 16: Baby's gender potentially visible on ultrasound. Baby approximately 4-5 inches, 3-4 oz. Week 20: Anatomy scan. Halfway point. The baby is approximately 6.5 inches, 10 oz. Week 24: Viability milestone. With NICU care, survival is possible (though with significant risks). Week 28: Third trimester begins. Baby approximately 14 inches, 2.5 lbs. Week 32: Baby's lungs nearly mature. Gaining approximately 0.5 lb per week. Week 36: Early term. Baby approximately 18 inches, 6 lbs. Week 37: Full term begins. Safe to deliver from this point. Week 40: Due date. Only 5% of babies born on exact due date.
IVF Due Date Calculation - Different from Natural Conception
IVF due date calculation differs from natural conception because the exact embryo age at transfer is known. Formula: Due date equals transfer date plus (266 minus embryo age in days). Day 3 transfer: add 263 days. Day 5 transfer (most common—blastocyst): add 261 days. Day 6 transfer: add 260 days. Example: A day 5 embryo transferred March 1, 2025. The due date equals March 1 plus 261 days equals November 18, 2025. Why 266 days? Natural conception produces a due date 266 days (38 weeks) after conception. Adding 14 days gives the standard 280-day LMP calculation. For IVF, we subtract the embryo age because the embryo has already been developing outside the body for 3-6 days. IVF pregnancies are monitored more closely—additional ultrasounds in the first trimester are standard to confirm development and update dating if needed.
Frequently Asked Questions About Pregnancy Due Dates
Q: How accurate is the due date calculator?
A: Due date calculators based on LMP give an estimated due date within a 2-week window for most women with regular 28-day cycles. The first-trimester ultrasound (7-12 weeks) is more accurate, with a margin of error of plus or minus 5-7 days. Second trimester ultrasound accuracy drops to plus or minus 2 weeks. Due dates are estimates—only 5% of babies are born on the exact due date. 90% are born within 2 weeks of the estimated due date.
Q: What if my cycles are irregular?
A: Irregular cycles make LMP-based calculation less accurate. If cycles vary by more than 7 days month to month, first trimester ultrasound dating is recommended over LMP calculation. Ultrasound at 8-10 weeks (crown-rump length measurement) is the gold standard for dating when LMP is uncertain. Our calculator allows cycle length adjustment (21-35 days) to improve accuracy for non-28-day cycles.
Q: What is a full-term pregnancy?
A: "Full term" is defined as 39-40 weeks by the American College of Obstetricians and Gynecologists (ACOG) since 2013. Early term: 37-38 weeks 6 days. Full term: 39-40 weeks 6 days. Late term: 41-41 weeks 6 days. Post-term: 42 weeks or more. The previous definition of "term" as any birth at 37+ weeks was changed because outcomes differ significantly between 37-week and 40-week births.
Q: How is IVF due date different from a natural pregnancy?
A: Natural pregnancy LMP adds 280 days. IVF adds 266 days minus embryo age at transfer (3, 5, or 6 days). A day 5 blastocyst transfer adds 261 days from transfer. IVF due dates are more precise because exact conception timing is known. Both methods aim to estimate the same endpoint: 38 weeks of fetal development.
Q: What is the viability milestone?
A: Fetal viability is generally considered to begin at 24 weeks. With intensive NICU care, survival is possible from 24 weeks, though rates and outcomes improve significantly with each additional week: 24 weeks, approximately 50% survival; 25 weeks, approximately 67%; 26 weeks, approximately 79%; 28 weeks, approximately 90%+, 32 weeks, approximately 98%+. Long-term disability rates also decrease significantly with gestational age at birth.
Q: Can I calculate the due date from the conception date?
A: Yes. If you know your approximate conception date (ovulation date), add 266 days (38 weeks) to get the due date. Ovulation typically occurs 14 days after the start of a 28-day cycle. So the conception date approximately equals LMP plus 14 days. Example: Conceived April 15, 2025. The due date equals April 15 plus 266 days equals January 7, 2026. Our calculator accepts conception date directly in the "By Conception Date" tab.