Pregnancy dating is one of the first numbers every expectant parent learns — and one of the most misunderstood. A pregnancy calculator estimates your due date and current week of pregnancy from your last menstrual period (LMP), the standard clinical method, and maps out your trimesters on a week-by-week schedule.
Most pregnancies last about 280 days (40 weeks) from the first day of the last menstrual period. Only about 4% of babies are born exactly on their estimated due date, and most are born between 38 and 42 weeks — so the due date is best understood as an estimate around which care is planned, not a promise.
How Pregnancy Dating Works
The standard calculation counts from the first day of your last menstrual period: add 280 days (40 weeks), the average gestational age from LMP to delivery. Clinicians often use Naegele's rule as a shortcut — add 7 days to the LMP, subtract 3 months, add 1 year — which yields the same 40-week date.
Gestational age counts from the first day of your period, which precedes conception by roughly two weeks. Because ovulation typically occurs about 14 days after the period's start, at "4 weeks pregnant" the embryo is actually about 2 weeks old. This dating convention is universal in medicine, and ultrasound measurements confirm and refine the estimate in the first trimester.
Conception-date calculations (counting from ovulation) produce dates about 2 weeks earlier in gestational terms — a source of confusion when comparing due dates from different methods. This calculator follows standard LMP-based gestational dating used by providers.
Trimesters and Milestones
Pregnancy is divided into three trimesters. The first trimester (weeks 1-13) includes implantation and the development of all major organs; symptoms like nausea and fatigue peak here. The second trimester (weeks 14-27) is typically the most comfortable, with organ systems maturing and fetal movement becoming noticeable around weeks 18-22. The third trimester (weeks 28-40+) brings rapid growth and final maturation for birth.
Weekly milestones that parents commonly track: the heartbeat is usually detectable around 6 weeks, the first ultrasound often schedules between 8-12 weeks, anatomy scans occur around 18-22 weeks, gestational diabetes screening around weeks 24-28, and provider visits increase weekly after 36 weeks.
Because due dates are estimates, providers mark the "term" window: early term (37-38 weeks), full term (39-41 weeks), and late/post term (41+ weeks). Babies born at 39-40 weeks generally have the best outcomes, and induction is typically considered after 41 weeks.
Estimating Due Dates and Their Accuracy
Due-date accuracy is highest in the first trimester. An ultrasound crown-rump-length measurement between 8-13 weeks dates a pregnancy within about ±5 days; later scans are progressively less precise as fetal size varies more. This is why providers recalculate the date if early ultrasound disagrees with the LMP-based estimate by more than a week.
Cycle irregularity complicates LMP dating: women with cycles longer or shorter than 28 days, or unknown LMP dates, are better dated by ultrasound. Assisted reproduction (IVF) provides the most precise dating — the transfer date establishes gestational age within a day.
For planning purposes, view your due date as a 40-week reference point within a normal 38-42 week delivery window. The calculator's week display lets you see where you are today, what trimester you are in, and how far you have to go — useful for scheduling milestones, tests, and preparation.
Knowing how pregnancy is dated transforms a confusing calendar into a clear roadmap. Use this calculator to see your current week, trimester, and estimated due date, and understand that the due date is a carefully estimated reference within a normal delivery window. Share the estimate with your provider, and let ultrasound in the first trimester refine it. The countdown has a number — but your baby decides the final date.