Pregnancy Due Date Calculator
Calculate your due date and track fetal development
Due Date
Your Pregnancy
Fetal Development: Week 0
Calculate your due date to see fetal development information.
Medical Basis of Due Date Calculation
Due dates are calculated from either the first day of your last menstrual period or measured via ultrasound. The standard medical calculation adds 280 days (40 weeks) to the first day of your last period, assuming a 28-day cycle.
This calculation method, known as Naegele’s rule, has been used since the early 19th century. While straightforward, it has limitations as it assumes all women have identical cycle lengths and ovulation timing.
Calculation Methods Compared
Calculates 40 weeks (280 days) from the first day of your last period. Most accurate when your cycle is consistently 28 days with ovulation on day 14.
Adds 38 weeks (266 days) to the conception date. Accurate when conception timing is known precisely through fertility tracking or treatments.
Measures embryo/fetus size to determine gestational age. Most accurate in the first trimester when fetal growth is more uniform across pregnancies.
Pregnancy Timeline
Embryonic heart activity begins
All major organs formed
Movements felt, anatomy scan performed
Viability threshold reached
Lung maturation accelerates
Full term pregnancy
Critical Planning Factors
Due Date Window
A due date is not a deadline but a midpoint in a 5-week window when birth is likely (37-42 weeks). Medical providers monitor this window rather than focusing on a single date.
First vs. Later Pregnancies
First-time mothers deliver, on average, 5 days after their due date, while subsequent pregnancies tend to deliver closer to the calculated date.
Due Date Adjustments
First-trimester ultrasound measurements showing more than a 5-7 day difference from LMP-calculated dates typically result in due date adjustments.
Planning Flexibility
Plan work leave, travel, and postpartum help with flexibility. Full-term birth can occur anywhere from 3 weeks before to 2 weeks after the calculated date.
Fetal Development Milestones
All essential organs begun forming. Heart beats 150-170 times per minute. Facial features taking shape.
Fetus moves limbs and makes facial expressions. All body systems in place and will continue developing through pregnancy.
Anatomy scan typically occurs. Fetus can hear sounds and establish sleep-wake patterns. Movements noticeable.
Viability threshold reached. Chance of survival if born prematurely, though would require medical intervention.
Lungs developing surfactant needed for breathing. Rapid weight gain as fat deposits form under the skin.
Considered full term. Majority of organ systems mature enough for extra-uterine life with minimal intervention.