Due Date Calculator
Your Due Date
Key Milestones
Naegele's Rule: The 280-Day Pregnancy Timeline
A due date calculator is an essential tool for expectant mothers to estimate when their baby will be born. The calculation is based on the first day of your last menstrual period (LMP), conception date, or ultrasound measurements. Understanding your due date helps you prepare for your baby's arrival and track important pregnancy milestones throughout the 40-week journey.
The standard pregnancy duration is 280 days or 40 weeks from the first day of your last menstrual period. This calculation method, known as Naegele's Rule, was developed in the 1800s and remains the most widely used approach today. However, only about 5% of babies are born exactly on their due date, with most arriving within two weeks before or after. Use our pregnancy calculator for additional pregnancy tracking and our ovulation calculator for conception planning.
How Do You Calculate Your Due Date from LMP?
The due date calculator uses three primary methods to estimate your baby's arrival. The LMP method adds 280 days (40 weeks) to the first day of your last menstrual period. This assumes a regular 28-day cycle with ovulation occurring on day 14. If your cycle is longer or shorter, the calculator adjusts accordingly by factoring in your average cycle length.
The conception date method is more precise if you know exactly when conception occurred. Since conception typically happens during ovulation (about 14 days after LMP), this method adds 266 days (38 weeks) to the conception date. This approach is particularly useful for women who used fertility tracking or assisted reproductive technology.
LMP, Conception, and Ultrasound Dating Methods
Last Menstrual Period (LMP) Method: This is the most common approach used by healthcare providers. The calculator adds 280 days to the first day of your last period. For example, if your LMP was January 1st, your due date would be October 8th. This method works best for women with regular 28-day cycles.
Conception Date Method: If you know when conception occurred, this method provides a more accurate estimate. The calculator adds 266 days to the conception date. This is ideal for women who tracked ovulation or know the exact date of intercourse that led to pregnancy.
Ultrasound Dating Method: Early ultrasounds (performed between 8-13 weeks) are the most accurate way to determine gestational age. The calculator uses the ultrasound date and gestational age at that time to project your due date. Ultrasound dating is typically accurate within 3-5 days.
What Are the Three Pregnancy Trimesters?
First Trimester (Weeks 1-13): This critical period involves rapid cell division and organ formation. By week 8, all major organs have begun developing. Morning sickness typically peaks around week 9 and subsides by week 14. The first prenatal visit usually occurs between weeks 8-12, where you'll receive your official due date.
Second Trimester (Weeks 14-27): Often called the "honeymoon phase," this period brings increased energy and reduced nausea. You'll feel your baby's first movements (quickening) between weeks 16-25. The anatomy scan ultrasound occurs around week 20, revealing your baby's sex and checking development.
Third Trimester (Weeks 28-40): Your baby undergoes final growth and development. By week 37, your baby is considered full-term. Weeks 39-40 are optimal for delivery, as the baby's lungs and brain complete maturation. Most healthcare providers won't let pregnancy continue beyond 42 weeks due to increased risks. Track your pregnancy journey with our EDD calculator for estimated delivery date calculations.
Irregular Cycles and Due Date Accuracy
Several factors can affect the accuracy of your due date calculation. Irregular menstrual cycles make LMP-based calculations less reliable, as ovulation may not occur on day 14. First-time mothers tend to deliver slightly later than the calculated due date, while women who've had previous pregnancies often deliver earlier. Your baby's genetics, maternal age, and overall health also influence when labor begins naturally.
Cycle length variations significantly impact due date accuracy. Women with 35-day cycles ovulate around day 21, meaning conception occurs a week later than the standard calculation assumes. Conversely, women with 21-day cycles ovulate earlier. Our calculator adjusts for cycle length to provide a more personalized estimate.
When to Expect Your Baby's Arrival
While your due date provides a target, babies rarely arrive exactly on schedule. Studies show that only 5% of babies are born on their due date. About 70% of babies arrive within 10 days of the due date (either before or after), and 90% are born within two weeks of the estimated date. First-time mothers are more likely to deliver after their due date, with an average delivery occurring at 40 weeks and 5 days.
Healthcare providers consider pregnancy full-term between 37 and 42 weeks. Early term is 37-38 weeks, full term is 39-40 weeks, late term is 41 weeks, and post-term is 42+ weeks. Babies born at 39-40 weeks have the best health outcomes, which is why many doctors recommend waiting until at least 39 weeks for scheduled deliveries unless medical reasons require earlier intervention.
Using Your Due Date for Pregnancy Planning
Your due date serves as a planning tool for prenatal care, maternity leave, and baby preparations. Schedule your prenatal appointments based on your due date: monthly visits until week 28, bi-weekly visits from weeks 28-36, and weekly visits from week 36 until delivery. Plan to have your nursery ready by week 36, as babies can arrive early. Start maternity leave discussions with your employer around week 30, considering that you may deliver 2-3 weeks before or after your due date.
Important pregnancy tests and screenings are scheduled based on your due date. The first-trimester screening occurs between weeks 11-14, the anatomy scan at week 20, the glucose screening test at weeks 24-28, and Group B strep testing at weeks 35-37. Knowing your accurate due date ensures you don't miss these critical checkpoints.