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, screenings, and milestone tracking.
Last updated: May 2026
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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 is simple: weeks = floor(LMP_days / 7); remaining_days = LMP_days mod 7. Gestational age is conventionally written as W:D (e.g., 12:3 means 12 weeks and 3 days). Variables: lmpDays (the number of days elapsed since the first day of the LMP). This differs from fetal age (or 'post-conceptional age'), which is typically about 2 weeks less because it counts from the estimated day of conception rather than the LMP. Gestational age governs which prenatal screenings are offered (combined first-trimester screening at 11:0–13:6; anatomy scan at 18–22 weeks; oral glucose tolerance test at 24–28 weeks), when certain ultrasounds are performed, and how preterm or post-term birth is classified. The WHO classifications: extremely preterm <28 weeks, very preterm 28:0–31:6, moderate to late preterm 32:0–36:6, early term 37:0–38:6, full term 39:0–40:6, late term 41:0–41:6, post-term ≥42:0. Edge cases: the formula assumes you know the LMP precisely; with irregular cycles or uncertain LMP, ultrasound-derived gestational age (crown-rump length at 8–13 weeks) is more accurate and should override the LMP-based estimate when discrepancies exceed 7 days in the first trimester (ACOG/RCOG guidance).
How to use
Example 1: LMP was 98 days ago. Step 1: weeks = floor(98/7) = 14. Step 2: remaining days = 98 mod 7 = 0. Result: 14 weeks and 0 days, written 14:0. Verify: 14 weeks × 7 = 98 days — matches input exactly. At 14:0 you are in the second trimester, eligible for the combined first-trimester screening if not yet performed (which closes at 13:6). Example 2: LMP was 240 days ago. Step 1: weeks = floor(240/7) = 34. Step 2: remaining days = 240 mod 7 = 240 − 238 = 2. Result: 34 weeks and 2 days, written 34:2. Verify: 34 × 7 + 2 = 240 days — matches. At 34:2 you are 'moderate to late preterm' territory; full term begins at 37:0. Approximately 39 days remain until the EDD at 40:0 (280 − 240 = 40 days, accounting for the start-of-week offset).
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 measurement used by all healthcare providers worldwide. Fetal age (or embryonic age or 'post-conceptional age') counts from the estimated day of conception, which is typically about 14 days after the LMP. So at delivery, a baby born at 40 weeks gestational age is approximately 38 weeks in fetal age. All clinical milestones — viability thresholds (~24 weeks), screening windows, preterm-birth classifications, and induction-timing decisions — are expressed in gestational weeks, not fetal weeks. The distinction matters most in research, IVF, and embryology contexts; in routine prenatal care, 'weeks pregnant' always means gestational weeks unless otherwise specified.
How accurate is gestational age calculated from the last menstrual period?
LMP-based gestational age is reliable to within ±5–7 days when menstrual cycles are regular (around 28 days) and the LMP date is remembered with certainty. It can be off by 1–2 weeks for women with irregular cycles, recent hormonal contraceptive use, breastfeeding, or uncertain LMP recall. A first-trimester ultrasound (crown-rump length at 8–13 weeks) is accurate to within 5–7 days and is used to confirm or revise the LMP-based estimate; ACOG and RCOG guidelines recommend that ultrasound dating supersede LMP when they disagree by more than 7 days. After 14 weeks, ultrasound dating becomes progressively less precise (±2 weeks at second trimester, ±3 weeks at third trimester), so the earliest available estimate is rarely revised later. IVF pregnancies use the known embryo transfer date, which is the most accurate of all.
Why do doctors use weeks and days instead of months to describe pregnancy?
Weeks and days provide a precise, standardized 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. The nuchal translucency scan, for example, must be performed between 11:0 and 13:6 — a precision that 'three months' cannot capture. The W:D format also maps directly onto trimester boundaries (T1 ends at 13:6, T2 at 27:6) and onto the WHO preterm-birth classifications (very preterm <32:0, moderate to late preterm 32:0–36:6). This standardization enables consistent care across providers, hospitals, and countries.
What are common mistakes when calculating gestational age?
Counting from the date your last period ended rather than the date it began shifts the result by 3–7 days. Confusing fetal age (counted from conception) with gestational age (counted from LMP) creates a systematic 2-week error in either direction. Applying LMP-based math to women with irregular or non-28-day cycles produces unreliable estimates without cycle-length adjustment. Forgetting to perform the modulo operation gives only complete weeks without the day remainder (a 14:6 pregnancy reported as '14 weeks' loses 6 days of precision). Mixing up sums of weeks and days when adding intervals (e.g., '12:5 + 3 weeks' should give 15:5, not 15:2 or 16:5). Finally, not updating gestational age after ultrasound revision means later screenings and decisions are based on a stale estimate.
When should I NOT rely on LMP-based gestational age?
Use ultrasound dating instead of LMP if your cycles are irregular, you recently used hormonal contraception or are breastfeeding, you don't remember the LMP date confidently, or you have polycystic ovary syndrome (PCOS). For IVF pregnancies, the embryo transfer date is the gold standard — LMP math does not apply. Once a first-trimester ultrasound is performed, the ultrasound-derived gestational age supersedes the LMP-based number if they disagree by more than 7 days. For very preterm or post-term decisions (around 24 weeks or 42 weeks), clinical decision-making requires confirmed dating from the earliest available ultrasound, not an LMP estimate alone. After 22–24 weeks, dating becomes less reliable from any method — defer to your obstetric provider's records for any clinical decision. Finally, gestational age math does not apply to molar pregnancies, ectopic pregnancies, or other pathologic implantations where 'pregnancy' is not progressing normally.