Gestational Age Calculator
Converts the number of days since your last menstrual period into gestational age expressed as weeks and days. Use it at any point in pregnancy to know your exact stage for prenatal care scheduling.
About this calculator
Gestational age is the standard clinical measure of how far along a pregnancy is, always counted from the first day of the last menstrual period (LMP). The conversion formula is straightforward: Weeks = floor(LMP days / 7); Remaining Days = LMP days mod 7. So a gestational age is written as W:D (e.g., 12:3 means 12 weeks and 3 days). This differs from fetal age (counted from conception), which is typically 2 weeks less. Gestational age governs which prenatal screenings are offered, when certain ultrasounds are performed, and how a preterm or post-term birth is classified. A full-term pregnancy is between 39 weeks 0 days and 40 weeks 6 days.
How to use
Suppose your LMP was 98 days ago. Step 1: Weeks = floor(98 / 7) = 14 weeks. Step 2: Remaining days = 98 mod 7 = 0 days. Result: You are 14 weeks and 0 days pregnant (written 14:0). For another example — 105 days since LMP: floor(105/7) = 15 weeks, 105 mod 7 = 0 days → 15:0. For 110 days: floor(110/7) = 15 weeks, 110 mod 7 = 5 days → 15 weeks and 5 days.
Frequently asked questions
What is gestational age and how is it different from fetal age?
Gestational age counts from the first day of your last menstrual period and is the standard used by all healthcare providers worldwide. Fetal age (or embryonic age) counts from the estimated day of conception, which is typically about 14 days after the LMP. At delivery, a baby born at 40 weeks gestational age is approximately 38 weeks in fetal age. All clinical milestones — viability thresholds, screening windows, and delivery timing — are expressed in gestational weeks, not fetal weeks.
How accurate is gestational age calculated from the last menstrual period?
LMP-based gestational age is reliable when menstrual cycles are regular (approximately 28 days) and the LMP date is known with certainty. It can be off by 1–2 weeks for women with irregular cycles, recent hormonal contraceptive use, or uncertain LMP recall. A first-trimester ultrasound (crown-rump length measurement at 8–12 weeks) is accurate to within 5–7 days and is used to confirm or revise the LMP-based estimate. After 20 weeks, ultrasound dating becomes less precise, so the early estimate is rarely changed.
Why do doctors use weeks and days instead of months to describe pregnancy?
Weeks and days provide a precise, standardised language that aligns directly with developmental and clinical milestones. Monthly estimates are ambiguous because calendar months vary in length (28–31 days), making it difficult to time screenings, scans, or interventions consistently. For example, the nuchal translucency scan must occur between 11 weeks 0 days and 13 weeks 6 days — a precision that 'three months' cannot capture. The W:D format also maps directly onto trimester boundaries and WHO preterm birth classifications.