Due Date Calculator

Introduction

The Due Date Calculator estimates your baby's expected delivery date based on either your last menstrual period (LMP) or your conception date. Knowing your estimated due date helps you track pregnancy milestones, plan prenatal appointments, and prepare for the arrival of your baby. The estimated due date is the foundation of prenatal care scheduling and provides a timeline for monitoring fetal development throughout each trimester.

The standard method for calculating due dates is Naegele's rule, which adds 280 days (40 weeks) to the first day of the last menstrual period. This method assumes a regular 28-day menstrual cycle with ovulation occurring on day 14. When the conception date is known, such as through assisted reproductive technology or precise ovulation tracking, the due date can be calculated by adding 266 days (38 weeks) from conception, which accounts for the approximately two weeks between LMP and ovulation. In vitro fertilization (IVF) pregnancies may have even more precise dating based on embryo transfer date.

It is important to understand that the estimated due date is exactly what it sounds like: an estimate. Only about 5% of babies are born on their exact due date. Most pregnancies last between 37 and 42 weeks, with a full-term pregnancy considered to be between 39 and 40 weeks. The due date provides a general timeframe for the end of pregnancy rather than a precise prediction. Early ultrasound measurements, particularly in the first trimester, provide the most accurate dating and may adjust the due date calculated from LMP alone. First-trimester crown-rump length measurements are accurate to within 5-7 days.

How to Use

The Due Date Calculator is straightforward to use. Select your preferred calculation method and enter the relevant date.

1. Select Calculation Method — Choose whether to calculate based on your Last Menstrual Period (LMP) or your Conception Date. The LMP method is more commonly used because many women know when their last period started. The conception method is useful if you tracked ovulation or conceived through fertility treatments such as IUI or IVF.
2. Enter the Date — Input the relevant date using the date picker. For the LMP method, enter the first day of your last menstrual period. For the conception method, enter the estimated date of conception (ovulation date). Be as accurate as possible for the best results.
3. Review Your Due Date — The calculator displays your estimated due date and the number of days remaining. Use this information to plan your prenatal appointments, maternity leave, and preparations for your baby's arrival. Remember that this is an estimate and your actual delivery date may vary. The estimated due date is the foundation of prenatal care scheduling and provides a timeline for monitoring fetal development.

Formulas and Calculations

Reference Tables

Limitations

Cycle Irregularity — Naegele's rule assumes a regular 28-day cycle with ovulation on day 14. Women with longer or shorter cycles, irregular cycles, or conditions like PCOS may ovulate at different times, making the LMP-based calculation less accurate. For these women, first-trimester ultrasound provides a more reliable due date.

Ultrasound Dating Discrepancy — Due dates calculated from LMP may differ from those determined by early ultrasound. When there is a discrepancy, ultrasound dating is generally considered more accurate, especially when performed between 8 and 13 weeks of pregnancy. Healthcare providers typically use ultrasound dating to confirm or adjust the LMP-based due date.

Not a Guarantee — The estimated due date is not a guarantee of when delivery will occur. Only about 5% of births happen on the exact due date. A normal pregnancy can range from 37 to 42 weeks. The calculator provides a reference point for planning purposes rather than a precise prediction.

Singleton Pregnancy Assumption — This calculator assumes a singleton pregnancy. Multiple pregnancies (twins, triplets) typically have shorter gestational lengths and different due date considerations. Women carrying multiples should rely on their healthcare provider's recommendations for due date estimation.

Practical Tips

Schedule Your First Prenatal Visit — Once you receive your estimated due date, schedule your first prenatal appointment. Early prenatal care is associated with better pregnancy outcomes. Many healthcare providers schedule the first visit between 8 and 12 weeks of pregnancy.

Track Pregnancy Milestones — Use your due date to mark key pregnancy milestones on your calendar. Knowing when each trimester begins and when important screenings occur helps you stay organized and prepared throughout your pregnancy journey.

Prepare for Flexibility — While having a due date is helpful for planning, remain flexible about the actual delivery date. Pack your hospital bag by 36 weeks and have your birth plan ready, but understand that your baby may arrive earlier or later than expected.

Discuss with Your Healthcare Provider — Share your calculated due date with your healthcare provider, who may adjust it based on ultrasound measurements and clinical assessment. Your provider's dating is the official date used for your medical records and care planning.

Frequently Asked Questions

How does Naegele rule calculate my due date?

Naegele rule estimates due date by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). Formula: LMP + 1 year - 3 months + 7 days.

How accurate is the due date calculated by LMP versus conception date?

Conception date calculation is more accurate because it does not rely on a 28-day cycle assumption. If you know ovulation or conception date, adding 266 days (38 weeks) gives a more precise estimate.

What percentage of babies are born on their exact due date?

Only about 4-5% of babies are born on their exact due date. The due date is an estimate of when you will be 40 weeks pregnant. Most babies arrive between 38 and 42 weeks.

Can irregular cycles affect my due date calculation?

Yes. Naegele rule assumes a 28-day cycle with ovulation on day 14. For irregular cycles, an early ultrasound measurement (crown-rump length between 11-14 weeks) is the most accurate way to date a pregnancy.

Why might my healthcare provider adjust my due date after an ultrasound?

Ultrasound in the first trimester can date a pregnancy within 5-7 days of accuracy. If the ultrasound differs from LMP-based date by more than 7 days, providers typically adjust to the ultrasound date.

References

1. Baskett, T.F., & Nagele, F. (2000). Naegele's rule: a reappraisal. BJOG: An International Journal of Obstetrics and Gynaecology, 107(11), 1433-1435.
2. Committee on Obstetric Practice. (2017). Methods for Estimating the Due Date. ACOG Committee Opinion No. 700. Obstetrics and Gynecology, 129(5), e150-e154.
3. Savitz, D.A., Terry, J.W., Dole, N., Thorp, J.M., Siega-Riz, A.M., & Herring, A.H. (2002). Comparison of pregnancy dating by last menstrual period, ultrasound scanning, and their combination. American Journal of Obstetrics and Gynecology, 187(6), 1660-1666.
4. Middleton, P., Shepherd, E., & Crowther, C.A. (2017). Induction of labour for improving birth outcomes for women at or beyond term. Cochrane Database of Systematic Reviews, (5).