Printed on 3/17/2026
For informational purposes only. This is not medical advice.
The Gestational Age Calculator determines the current gestational age in weeks and days from the first day of the last menstrual period (LMP). It also calculates the estimated due date (EDD) using Naegele's rule (LMP + 280 days). Gestational age (GA) is the standard measure used throughout pregnancy to track fetal development, schedule appropriate screening tests, and plan delivery timing. The calculator identifies the current trimester: first (0–12 weeks), second (13–26 weeks), or third (27–40+ weeks). Also called age of gestation (AOG) or estimated gestational age (EGA). For full due date calculation see [Pregnancy Due Date Calculator](/tools/pregnancy-due-date).
Formula: Gestational Age = days since LMP. EDD = LMP + 280 days (Naegele's rule).
The LMP is the first day of your most recent period before pregnancy. This is the universal starting point for gestational age — even though conception occurs about 2 weeks later, all standard pregnancy dating counts from the LMP. If you are unsure of the exact date, use the closest date you can recall.
The calculator counts the total days from LMP to today, then divides into weeks and remaining days. For example, 65 days from LMP = 9 weeks and 2 days (9w2d). Simultaneously, it adds 280 days (40 weeks) to the LMP to calculate the estimated due date (EDD) — this is Naegele's rule, the international standard for pregnancy dating.
Your results show current gestational age in weeks and days, the estimated due date (EDD), and the current trimester (first, second, or third). Use this to plan prenatal appointments, schedule time-sensitive screening tests, and track fetal developmental milestones week by week.
Newly pregnant patients
Determine gestational age and estimated due date at the first prenatal appointment. Accurate dating is essential for scheduling the nuchal translucency screen (11–13 weeks), first-trimester combined screening, and anatomy scan timing. LMP-based dating is then confirmed by first-trimester ultrasound.
OB/GYN physicians, midwives, nurses
Quickly establish gestational age in any clinical encounter. Gestational age determines fetal viability thresholds (23–24 weeks), appropriate screening schedules, and delivery timing in high-risk pregnancies. Use alongside Fetal Weight Calculator to assess growth appropriateness.
Pregnant patients and providers
Time-sensitive prenatal tests require specific gestational age windows: nuchal translucency (11w0d–13w6d), quad screen (15–20 weeks), anatomy scan (18–22 weeks), glucose tolerance test (24–28 weeks), and Group B Strep screening (36–37 weeks). Missing these windows can delay or invalidate results.
Labor and delivery teams
Determine gestational age in the setting of preterm labor or preterm premature rupture of membranes (PPROM). Gestational age drives decisions about antenatal corticosteroids (23–34 weeks), magnesium neuroprotection (less than 32 weeks), tocolysis, and level of nursery preparation needed.
Working parents and HR
Calculate the expected due date to plan maternity leave, paternity leave, and workplace coverage arrangements. Most leave policies are triggered by the due date or actual birth date — knowing gestational age helps project these timelines months in advance.
Expectant mothers
Track week-by-week pregnancy milestones, understand which trimester you are in, and see how many days remain until the due date. Each gestational week corresponds to specific fetal developmental milestones — from organogenesis in the first trimester to lung maturation in the third.
Gestational age is counted from the first day of the last menstrual period — the day bleeding starts, not the day it stops. Many patients confuse this and use the last day of their period or the day they noticed they were pregnant, both of which produce an inaccurate gestational age.
If a first-trimester ultrasound crown-rump length measurement differs from your LMP date by more than 7 days, ACOG recommends using the ultrasound date. This is because recalled LMP dates are often off by several days, while a 7–10 week ultrasound is accurate to within 5 days. Always ask your provider which dating method was used.
Age of gestation (AOG) and gestational age (GA) are interchangeable terms used in different countries and medical traditions. The Philippines commonly uses AOG, while the US and UK use GA. The calculation is identical — both count from the first day of the last menstrual period.
Only about 5% of babies are born on their exact due date, but approximately 80% arrive within two weeks of it. A pregnancy is considered full term at 39w0d–40w6d, early term at 37–38 weeks, and post-term beyond 42 weeks. Deliveries before 37 weeks are classified as preterm regardless of fetal size.
Pregnancies conceived through IVF use the embryo transfer date, not the LMP, for dating. A day-5 blastocyst transfer is equivalent to LMP + 19 days. If you conceived through IVF, ask your fertility clinic for the gestational age equivalent — do not use your most recent natural period for this calculation.
Your result shows the current gestational age in weeks and days, the estimated due date (EDD), and the current trimester. Gestational age is counted from the first day of the last menstrual period (LMP), which means it includes approximately two weeks before conception actually occurred. A pregnancy at 8 weeks gestational age, for example, represents roughly 6 weeks since fertilization.
The trimester breakdown is clinically important: the first trimester (0-12 weeks) is the period of organogenesis when the fetus is most vulnerable to teratogens; the second trimester (13-26 weeks) is when anatomy scans and screening tests are typically performed; and the third trimester (27-40+ weeks) focuses on fetal growth monitoring and delivery planning. Each trimester has specific milestones, screening tests, and potential complications that clinicians track.
The estimated due date represents 40 weeks (280 days) from the LMP. Only about 5% of babies are born on their actual due date, but approximately 80% are born within two weeks of it. A pregnancy is considered full term between 39 weeks 0 days and 40 weeks 6 days, early term at 37-38 weeks, and post-term beyond 42 weeks.
Use this calculator at any point during pregnancy when you need to determine the current gestational age or expected due date based on the last menstrual period. It is particularly useful at the first prenatal visit to establish dating, which is then confirmed or adjusted by first-trimester ultrasound. Accurate gestational age is essential for scheduling time-sensitive prenatal tests such as the nuchal translucency screen (11-13 weeks), anatomy scan (18-22 weeks), glucose tolerance test (24-28 weeks, manage with [TDEE Calculator](/tools/tdee-calculator) and [Macro Calculator](/tools/macro-calculator) for gestational diabetes), and Group B Strep screening (36-37 weeks).
Clinicians also use gestational age to determine fetal viability thresholds, guide decisions about delivery timing (assess readiness with [Bishop Score](/tools/bishop-score)) in complicated pregnancies, and assess whether fetal growth is appropriate (track maternal weight with [BMI Calculator](/tools/bmi-calculator)). Patients often find it helpful for tracking pregnancy milestones and understanding which developmental stage their baby has reached.
LMP-based gestational age assumes a regular 28-day menstrual cycle with ovulation on day 14. For women with irregular cycles, cycles longer or shorter than 28 days, or uncertain LMP dates, the calculated gestational age may be inaccurate by one to several weeks. In these cases, first-trimester ultrasound measurement of crown-rump length is more reliable and should be used for dating.
If the ultrasound-based date differs from the LMP-based date by more than 7 days in the first trimester, more than 10 days in the second trimester, or more than 21 days in the third trimester, the ultrasound date is generally adopted as the official estimated due date per ACOG guidelines. This calculator does not incorporate ultrasound data and relies solely on LMP.
The calculator also does not apply to pregnancies conceived through assisted reproductive technology (IVF/ICSI), which have known conception dates and should be dated from the date of embryo transfer. Women who were recently using hormonal contraception may have delayed ovulation after discontinuation, making LMP-based dating unreliable for the first cycle. For high-risk pregnancies with obesity (check [BMI](/tools/bmi-calculator)) or cardiovascular disease (assess with [ASCVD Risk](/tools/ascvd-risk)), accurate gestational dating becomes even more critical for monitoring.
Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with questions about your health.
Calculate your estimated due date (EDD) from your last menstrual period using Naegele's rule. Track gestational age and trimester.
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