Gestational Age Calculator

Pregnancy weeks do not start when sperm meets egg. They start two weeks earlier, on the first day of the period that preceded conception. This seems counterintuitive, but it exists for a practical reason: most people do not know their exact ovulation date, while the start of a period is usually remembered or recorded. Counting from LMP gives every provider a shared, consistent reference point.

The calculation is straightforward: count the number of days from LMP to today, divide by seven to get full weeks, and take the remainder as extra days. A standard pregnancy runs 280 days from LMP — exactly 40 weeks. A cycle length adjustment shifts the due date by the difference between your cycle and 28 days. If your cycle is 32 days, your due date is 4 days later than the standard Naegele's Rule result.

Trimester boundaries are based on gestational weeks. The first trimester covers weeks 1 through 12, the second spans weeks 13 through 26, and the third runs from week 27 to delivery. These boundaries matter clinically because screening tests, safe medication windows, and intervention options differ across each trimester. Knowing your trimester before an appointment means you can ask the right questions.

Use this calculator when you have a confirmed or likely LMP date and want to know your current gestational age before or between prenatal appointments. It is useful for confirming which trimester you are in, understanding which screenings are coming up, and planning time-sensitive decisions like prenatal testing windows.

This tool is also appropriate when comparing your tracking app result to what a provider told you — discrepancies sometimes arise from different cycle length assumptions or from the provider using an ultrasound-corrected date rather than LMP.

Do not rely on this tool as a substitute for ultrasound dating, especially in the first trimester when crown-rump length measurement gives a more accurate gestational age than any date-based formula. If your ultrasound date and LMP-based date differ by more than 7-10 days, your provider will typically use the ultrasound date for all subsequent calculations. This calculator cannot account for irregular ovulation, assisted reproductive technology timelines, or known implantation dates.

The most common mistake is confusing gestational weeks with weeks since conception. A person who conceived 6 weeks ago is 8 weeks gestational age. Entering conception date instead of LMP into any LMP-based calculator will produce a result that is 2 weeks too low — which can trigger unnecessary concern about growth or dating discrepancy.

A second mistake is using the end of a period rather than the first day. Periods last 3-7 days, and only the first day is the correct reference point. Using day 5 of a period as the LMP can shift the due date by nearly a week and misrepresent the current gestational age by the same margin.

A third mistake applies to irregular cycles: assuming the default 28-day calculation is close enough. For someone with a 40-day cycle, the default calculation places ovulation on day 14 when it likely occurred around day 26 — a 12-day error that affects both the gestational age reading and the due date. Entering your actual average cycle length is a simple correction that materially improves accuracy.

The core formula is: gestational age in days = today minus LMP date. Weeks equals floor(days divided by 7). Remainder days equals total days modulo 7.

Due date calculation uses Naegele's Rule as a baseline: add 280 days to LMP. For non-standard cycles, the adjustment is: due date = LMP + 280 + (cycle length minus 28). A 35-day cycle produces a due date of LMP + 287. A 21-day cycle produces LMP + 273.

Conception date estimation works backward from ovulation: conception day = LMP + (cycle length minus 14). This assumes ovulation occurs 14 days before the next expected period, which holds reasonably well for regular cycles. For irregular cycles or confirmed ovulation tracking data, a fertility specialist can provide a more precise estimate. No formula based on LMP alone can determine conception to the exact day — only ultrasound biometry or known insemination dates can do that.

The 28-day cycle assumption embedded in standard gestational age formulas systematically miscalculates due dates for the roughly 30% of people with cycles outside the 26-30 day range. More importantly, the formula treats ovulation timing as a fixed offset from LMP when it is actually variable even within the same individual across cycles. Ultrasound crown-rump length at 8-12 weeks is accurate to plus or minus 5 days and is the clinical gold standard precisely because it measures actual fetal development rather than inferring it from menstrual history. Any LMP-based calculator — including this one — produces a planning estimate, not a clinical dating result.