Gestational Age Calculator

Pregnancy timing works backward from a known reference point. Clinicians settled on the first day of the last menstrual period as the universal anchor because it is an observable, recordable date — whereas the actual moment of fertilization is almost never known outside a fertility clinic. This means every pregnancy is dated as if conception happened two weeks later than the LMP, because ovulation typically falls around day 14 of a 28-day cycle.

The result is a counting system that feels slightly counterintuitive: at the moment of conception, a person is already considered about two weeks pregnant by clinical standards. By the time a positive home pregnancy test appears — usually around week 4 — two of those four weeks predate fertilization entirely. This matters because gestational age gates prenatal screening windows, medication safety categories, and delivery planning.

When you enter a conception date instead of an LMP, this tool performs the reverse step: it subtracts 14 days from your conception date to establish the equivalent LMP anchor, then counts forward from there. The result matches what your provider would calculate using the standard LMP method, keeping your gestational age consistent across all your care records.

Use this tool when you want to know your current gestational age in weeks and days, confirm your estimated due date, or find out which trimester you are in. It is appropriate from the moment you have a confirmed LMP or conception date through the end of pregnancy. It is also useful for midwives, doulas, and birth partners who want to quickly verify a patient's stage of pregnancy ahead of an appointment.

This tool is less appropriate when your cycle is significantly irregular (shorter than 21 days or longer than 35 days), because the LMP method assumes a standard 28-day cycle with ovulation on day 14. In irregular cycles, LMP-based dating can be off by a week or more in either direction. In that case, a first-trimester ultrasound is more reliable and your provider may formally change your due date after reviewing the scan measurements.

Do not use LMP-based gestational age as a substitute for clinical dating when the stakes are high — for example, when deciding about amniocentesis timing, preterm labor management, or post-dates induction scheduling. For those decisions, ultrasound-confirmed gestational age from your care provider is the authoritative source.

Entering the wrong LMP date. The most common error is entering the date of a missed period, a positive test, or the last sexual contact rather than the first day of the last menstrual bleed. The calculation anchors entirely to that one date, so a week's error in LMP produces a week's error in gestational age and due date. Check your calendar app or period-tracking app before entering.

Confusing conception date with implantation or test date. When using conception-date mode, users sometimes enter when they believe implantation occurred (typically 6- 12 days after ovulation) or when they got a positive test result. Neither is the conception date. Conception is the date of the sexual encounter or, for IVF, the date of egg retrieval or embryo transfer. Entering a date 10 days late shifts the entire calculation by 10 days.

Assuming the due date is exact. The 280-day term is a population average, not an individual prediction. Cycle length, age, parity, and other factors all affect actual delivery timing. This result gives you a planning anchor and a gestational age for scheduling appointments — it does not predict the actual birth date. Decisions about induction, travel, or work leave should account for the full range of normal delivery, not just the single due date.

The core calculation has two steps: establish the LMP anchor date, then count the days elapsed. For LMP entry, the anchor is the date you enter directly. For conception-date entry, the anchor is your conception date minus 14 days.

Total gestational days = (today's date) minus (LMP anchor date), measured in whole days. Gestational weeks = floor(total gestational days divided by 7). Remaining days = total gestational days modulo 7. So if the total is 129 days, that gives 18 complete weeks plus 3 additional days, displayed as 18 weeks, 3 days.

The estimated due date adds 280 days to the LMP anchor. That is the origin of Naegele's rule, which clinicians have used since the nineteenth century: take the LMP date, add 280 days, and you have the estimated due date. The trimester boundaries divide 280 days into three segments: days 1 through 91 are the first trimester, days 92 through 196 are the second, and days 197 onward are the third, ending at 280 days.

Naegele's rule assumes a fixed 280-day term and a 28-day cycle, but neither is universally true. Studies of spontaneous labor onset show the median term is closer to 283 days for first pregnancies, and the standard deviation is roughly 8 days — meaning the bell curve of delivery dates spans more than three weeks on each side of the calculated due date. The 280-day convention survives because it is reproducible and clinically convenient, not because it predicts individual outcomes.

The ovulation-offset assumption embedded in the LMP method also breaks down at the population tails. Luteal phase defects, polycystic ovary syndrome, and perimenopause all shift the LMP-to-ovulation window. When conception-date mode is used with a known IVF date, the 14-day subtraction is still applied — but the true biological anchor is the egg retrieval date, and the LMP-equivalent result is a mathematical convention rather than a biological reality. Practitioners using this for IVF patients should understand they are working within a shared clinical fiction that keeps records consistent across natural and assisted conceptions.