Pregnancy Due Date Calculator
Estimate when your baby will arrive using Naegele's rule with your LMP or conception date
Your Estimated Due Date
Trimester Milestones & Nutrition
Key developmental milestones and nutrition focus for each trimester
- Heart begins to beat by week 6
- Brain and spinal cord forming
- Arms, legs, fingers, and toes develop
- All major organs begin to form by week 10
- Baby begins to move and kick (around week 18-20)
- Sex can be determined via ultrasound
- Hearing develops; baby responds to sound
- Rapid growth in length and weight begins
- Lungs mature and practice breathing motions
- Baby gains about half a pound per week
- Brain develops rapidly in the final weeks
- Baby typically moves into head-down position by week 36
Understanding Pregnancy Due Dates
A pregnancy due date, also called the estimated date of delivery (EDD), is the projected date when a baby is most likely to be born. The most widely used method for calculating a due date is Naegele's rule, developed by German obstetrician Franz Karl Naegele in the early 19th century. The rule works by adding 280 days (40 weeks) to the first day of the woman's last menstrual period (LMP). This 40-week timeframe assumes a standard 28-day menstrual cycle with ovulation occurring on day 14. Since conception typically occurs around ovulation, the actual gestational development of the baby spans approximately 266 days (38 weeks) from conception. Most healthcare providers use the LMP method because the exact date of conception is often unknown, whereas women generally remember the start of their last period.
Cycle length plays an important role in due date accuracy. If your average menstrual cycle is longer than 28 days, ovulation likely occurs later, which means the due date should be shifted forward by the number of extra days. Conversely, a shorter cycle suggests earlier ovulation and an earlier due date. For example, a woman with a 35-day cycle ovulates approximately 7 days later than someone with a 28-day cycle, so her adjusted due date would be 7 days later than the standard Naegele's calculation. This adjustment is straightforward: the difference between the actual cycle length and 28 days is simply added to or subtracted from the standard EDD. First-trimester ultrasound measurements, particularly the crown-rump length, can further refine the estimate and are considered the most accurate dating method available, usually reliable to within five to seven days.
Only about 4 to 5 percent of babies are born exactly on their estimated due date. The majority of births occur within a two-week window around the EDD, between 38 and 42 weeks of gestation. A pregnancy is considered full term at 39 weeks, early term between 37 and 38 weeks, and post-term after 42 weeks. Factors that influence when labor begins include the mother's age, whether it is a first or subsequent pregnancy, genetics, stress, physical activity, and the baby's position. First-time mothers tend to deliver slightly later than the EDD on average, while women who have given birth before often deliver a few days earlier. Twins and multiples tend to arrive earlier, often around 36 to 37 weeks.
Understanding your due date helps you and your healthcare team plan prenatal care, schedule important screenings and tests, and prepare for delivery. Key milestones tied to the due date include the first-trimester screening (weeks 11 to 13), the anatomy scan ultrasound (weeks 18 to 22), the glucose tolerance test for gestational diabetes (weeks 24 to 28), Group B Streptococcus testing (weeks 35 to 37), and discussions about birth plans and labor induction if the pregnancy extends past 41 weeks. While the due date is an estimate and not a precise prediction, it serves as the central reference point around which all prenatal care is organized. Always consult your obstetrician or midwife for personalized guidance based on your specific circumstances.
How to Use This Pregnancy Due Date Calculator
- Select your input method using the toggle at the top: "Last Menstrual Period" if you know when your last period started, or "Conception Date" if you know when you conceived or ovulated.
- Enter the relevant date. For the LMP method, enter the first day of your last menstrual period. For the conception method, enter your estimated conception or ovulation date.
- Adjust the average cycle length if your cycle is not the standard 28 days. This is only used with the LMP method and refines the calculation by accounting for when you likely ovulated.
- Click "Calculate Due Date" to generate your results. You will see your estimated due date displayed as a calendar card, a countdown to delivery, your current week and trimester, and a visual pregnancy timeline.
- Scroll down to explore key pregnancy dates, trimester milestones with developmental stages, and nutrition tips for each phase. Share these results with your healthcare provider to compare with ultrasound dating.
Frequently Asked Questions
How accurate is a pregnancy due date calculator?
A due date calculated from the last menstrual period using Naegele's rule is accurate to within about two weeks for most women. Only 4 to 5 percent of babies are born on the exact due date, but approximately 80 percent are born within the window of 37 to 42 weeks. First-trimester ultrasound is considered the most accurate dating method, reliable to within 5 to 7 days. If there is a discrepancy of more than 7 days between the LMP-based date and an ultrasound date in the first trimester, most providers will use the ultrasound date as the official EDD.
What is Naegele's rule and how does it work?
Naegele's rule is a standard formula for estimating the due date. It adds 280 days (40 weeks) to the first day of the last menstrual period. An equivalent shortcut is to add 7 days to the LMP date and then subtract 3 months. The rule assumes a 28-day menstrual cycle with ovulation on day 14. For cycles that differ from 28 days, the calculation is adjusted by adding or subtracting the difference. For instance, if your cycle is 32 days, you add 4 extra days to the standard result because ovulation occurs later in a longer cycle.
Does cycle length affect my due date?
Yes, cycle length directly affects the due date calculation. Naegele's rule assumes a 28-day cycle, but many women have cycles ranging from 21 to 35 days or more. If your cycle is longer than 28 days, you ovulate later, which means conception occurs later and the due date should be pushed forward by the number of extra days. A shorter cycle means earlier ovulation and an earlier due date. For example, with a 35-day cycle, the due date would be 7 days later than the standard calculation. This is why entering an accurate cycle length into the calculator produces a more reliable estimate.
What is the difference between gestational age and fetal age?
Gestational age counts from the first day of the last menstrual period, which is typically about two weeks before conception actually occurs. A full-term pregnancy is 40 weeks gestational age. Fetal age, also called embryonic age or conceptional age, counts from the actual date of conception and is about two weeks less than gestational age. So when a pregnancy is described as 12 weeks along, the fetus is actually about 10 weeks old. Healthcare providers and due date calculators use gestational age because the LMP is a more reliably known date than the exact moment of conception.
Can my due date change during pregnancy?
Yes, your due date may be revised based on ultrasound measurements, particularly if the first-trimester ultrasound shows a crown-rump length that differs significantly from what the LMP-based date predicts. If the ultrasound date and LMP date differ by more than 7 days in the first trimester, or more than 10 to 14 days in the second trimester, your healthcare provider will typically adopt the ultrasound date. The due date may also be adjusted if you are uncertain about your LMP or have very irregular cycles. Once established, however, the due date generally should not be changed based on later ultrasounds.
What counts as full term, early term, and post-term pregnancy?
The American College of Obstetricians and Gynecologists defines the following categories: Early term is 37 weeks 0 days through 38 weeks 6 days. Full term is 39 weeks 0 days through 40 weeks 6 days. Late term is 41 weeks 0 days through 41 weeks 6 days. Post-term is 42 weeks 0 days and beyond. Babies born at full term (39 to 40 weeks) generally have the best health outcomes. If a pregnancy extends past 41 weeks, healthcare providers often discuss labor induction to reduce risks associated with post-term pregnancy, such as decreased amniotic fluid and placental insufficiency.
Should I use LMP or conception date for a more accurate result?
If you know your exact conception or ovulation date with confidence, such as through ovulation tracking, fertility treatment, or IVF, using the conception date produces a more accurate due date because it removes the uncertainty about cycle length and ovulation timing. The conception method adds 266 days (38 weeks) to the conception date. However, most women do not know their precise conception date, so the LMP method is the standard approach. When using the LMP method, entering your actual average cycle length rather than the default 28 days will improve accuracy, especially if your cycles are consistently shorter or longer than 28 days.
