Pregnancy Week Calculator
Tells you exactly how many weeks pregnant you are based on your LMP, conception date, or due date, with an adjustment for cycle length. Perfect for confirming gestational age between prenatal visits.
About this calculator
Gestational age is universally measured in completed weeks from the first day of the last menstrual period (LMP), even though conception does not occur until roughly day 14 of a 28-day cycle. This calculator first computes the difference in days between the reference date and today: diffDays = currentDate − referenceDate. If a conception date is provided, 14 days are added to convert to gestational age (gestationalDays = diffDays + 14). If a due date is provided, gestational days are calculated as 280 − daysUntilDue. A cycle length adjustment (cycleAdjustment = cycleLength − 28) is then added to correct for cycles that differ from the assumed 28-day standard. Finally, gestational weeks = floor(gestationalDays / 7). The remainder of that division gives the extra days within the current week, so a result of 9 weeks and 3 days means you are in your 10th week of pregnancy.
How to use
Suppose your LMP was January 10, 2025, today is April 10, 2025, and your cycle is 30 days (cycleAdjustment = 30 − 28 = +2 days). Step 1: diffDays = 90 days (Jan 10 to Apr 10). Step 2: Using LMP method, no +14 addition needed. gestationalDays = 90 + 2 = 92. Step 3: gestational weeks = floor(92 / 7) = 13 weeks, with 1 extra day. Result: 13 weeks and 1 day pregnant. If your cycle were exactly 28 days, the answer would be 12 weeks and 6 days — a meaningful difference for clinical dating.
Frequently asked questions
What is the difference between calculating pregnancy weeks from LMP versus conception date?
The LMP method adds 14 days to conception-based counting because ovulation — and thus fertilization — typically occurs around day 14 of a 28-day cycle. Clinicians prefer LMP because the start of the last period is usually a known date, whereas the exact moment of conception almost never is. When a conception date is used (for example, from IVF records), 14 days are added to align the result with the gestational age scale that all pregnancy milestones and reference charts use. Both methods should give similar results when cycle length is close to 28 days.
How many weeks pregnant am I if I know my due date?
If you know your due date, you can work backward: a full-term pregnancy is 280 days (40 weeks) from LMP. The number of gestational days already elapsed equals 280 minus the number of days remaining until your due date. Divide by 7 and take the floor to get completed weeks. For example, if your due date is 70 days away, you have completed 280 − 70 = 210 days, which equals exactly 30 weeks. This calculator automates that arithmetic for you and applies any cycle length correction needed.
Why does cycle length matter when calculating how far along I am?
Standard pregnancy dating assumes a 28-day cycle with ovulation on day 14. If your cycles are longer — say 35 days — ovulation likely occurred around day 21, meaning conception happened about a week later than the standard model assumes. Without correction, your gestational age would be overstated by roughly 7 days. This calculator adds (cycleLength − 28) days to the gestational day count to correct for this offset. The adjustment is especially important for accurate first-trimester dating and for reconciling LMP-based estimates with ultrasound measurements.