How to use this calculator
Choose whether to calculate from your last menstrual period (LMP) or from a known conception date. If using LMP, enter the first day of your most recent period and your average cycle length. If you know the date of conception, select that option and enter the date directly.
The calculator uses Naegele's rule to estimate your due date, then displays a complete trimester timeline showing where you are in your pregnancy right now. You will see your current week, the dates for each trimester boundary, and key milestones like when the heartbeat becomes detectable or when the anatomy scan typically occurs.
Keep in mind that this tool provides an estimate. Your healthcare provider may adjust the date based on ultrasound measurements, especially during the first trimester. If your cycle length is significantly different from 28 days, entering the correct length will improve the accuracy of the estimate.
How due dates are calculated
The most common method is Naegele's rule, which adds 280 days (40 weeks) to the first day of your last menstrual period. This formula was developed in the early 1800s and remains the standard starting point for estimating due dates today.
The calculation assumes a 28-day menstrual cycle with ovulation happening around day 14. Since pregnancy is dated from the LMP rather than conception, the first two weeks of "pregnancy" actually occur before the egg is fertilized. If you know your conception date, subtracting those two weeks gives the same result: conception date plus 266 days.
Healthcare providers often refine the estimate with a first-trimester ultrasound. By measuring the embryo's crown-rump length, they can estimate gestational age within a few days. If the ultrasound date differs significantly from the LMP-based date, your provider may adjust your official due date.
What each trimester looks like
First trimester (weeks 1 to 13)
The fertilized egg implants in the uterus and major organs begin forming. By week 8 the heartbeat is detectable, and by week 12 the baby is about two inches long. Many parents experience morning sickness and fatigue during this period, and the risk of miscarriage drops significantly after week 12.
Second trimester (weeks 14 to 27)
Often called the "golden trimester," many parents feel more energetic during this stage. The baby grows rapidly, kicks become noticeable around weeks 18 to 22, and the detailed anatomy scan usually happens around week 20. By the end of this trimester the baby is roughly 14 inches long and weighs about two pounds.
Third trimester (weeks 28 to 40)
The baby gains weight quickly, the lungs mature, and the brain develops rapidly. By week 37 the baby is considered early term. Most babies settle into a head-down position in preparation for birth. Full term is 39 to 40 weeks, though healthy babies can arrive anywhere between 37 and 42 weeks.
Frequently asked questions
How is a due date calculated from the last menstrual period?
Due dates are most commonly calculated using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the estimate may be adjusted by your healthcare provider.
How accurate is a due date calculator?
A due date calculator provides an estimate. Only about 5% of babies are born on the exact due date. Most births occur within two weeks before or after the estimated date. An early ultrasound before 12 weeks can provide a more precise estimate.
What is the difference between calculating from LMP vs. conception date?
Calculating from the LMP adds 280 days because it includes about two weeks before conception. Calculating from the conception date adds 266 days since it starts from fertilization. Both methods should produce the same due date if conception occurred roughly 14 days after the LMP.
Can my due date change during pregnancy?
Yes. Your due date can be adjusted based on ultrasound measurements, especially during the first trimester. If there is a discrepancy of more than 7 days between the LMP-based date and the ultrasound, your provider may update the due date for more accurate prenatal care planning.