Health & Medicine · Obstetrics & Pediatrics
Gestational Age Calculator
Calculates gestational age in weeks and days from the last menstrual period (LMP) or estimated due date (EDD), and estimates the expected delivery date.
Calculator
Format: YYYY-MM-DD
Formula
GA (days) is the gestational age in total days, calculated as the difference between today's date and the LMP date. The Estimated Due Date (EDD) is determined by adding 280 days (40 weeks) to the LMP, following Naegele's Rule. Gestational age in weeks is the integer quotient of total days divided by 7, and the remainder gives the extra days beyond completed weeks.
Source: Naegele's Rule — Naegele FC, 1812; American College of Obstetricians and Gynecologists (ACOG) Practice Bulletin No. 700, 2017.
How it works
Gestational age is the standard measure of pregnancy duration, counted from the first day of the last menstrual period rather than from the actual date of conception. This convention is used because the LMP is a reliably known date, while ovulation and fertilization are often uncertain. A full-term pregnancy spans approximately 40 weeks (280 days) from the LMP, though normal deliveries can occur between 37 and 42 weeks.
The primary formula used is Naegele's Rule: the Estimated Due Date (EDD) is calculated by adding 280 days to the LMP date. Gestational age on any given day is calculated as the difference in days between today's date and the LMP, then converted to weeks and days. For example, 196 days from the LMP corresponds to exactly 28 weeks and 0 days of gestation. The trimester is determined by completed gestational weeks: the first trimester spans weeks 0–12, the second trimester weeks 13–26, and the third trimester weeks 27–40+.
Clinicians use gestational age to time routine prenatal visits, order appropriate laboratory tests and ultrasounds, administer vaccinations (such as Tdap), interpret fetal biometry measurements, and assess risk for preterm labor or post-term pregnancy. In cases where LMP is uncertain or irregular cycles exist, ultrasound-based dating (crown-rump length in the first trimester) provides a more accurate estimate and may be used to adjust the EDD.
Worked example
Suppose a patient's last menstrual period began on September 1, 2024, and today's date is January 15, 2025.
Step 1 — Calculate total gestational days:
From September 1, 2024 to January 15, 2025 = 136 days.
Step 2 — Convert to weeks and days:
136 ÷ 7 = 19 weeks and 3 days.
The patient is therefore at 19 weeks and 3 days of gestation.
Step 3 — Calculate the Estimated Due Date (EDD):
September 1, 2024 + 280 days = June 7, 2025.
Step 4 — Identify the trimester:
19 weeks falls within the second trimester (weeks 13–26).
This information informs the provider to schedule an anatomy ultrasound (typically performed between 18–22 weeks), review quad-screen results if ordered, and counsel the patient regarding fetal movement milestones.
Limitations & notes
This calculator is based on Naegele's Rule, which assumes a regular 28-day menstrual cycle with ovulation occurring on day 14. Women with irregular cycles, polycystic ovary syndrome (PCOS), recent hormonal contraceptive use, or unknown LMP dates may have inaccurate gestational age estimates using this method alone. In such cases, first-trimester ultrasound crown-rump length (CRL) measurement is the gold standard for dating and should be used to adjust the EDD. Additionally, this tool is intended for educational and informational purposes and does not replace clinical evaluation by a qualified healthcare provider. Pregnancy dating should always be confirmed and managed by a licensed obstetrician or midwife. Results are calculated based on the current system date and may vary if used on different devices or time zones.
Frequently asked questions
What is gestational age and how is it measured?
Gestational age is the duration of a pregnancy measured in weeks and days, starting from the first day of the woman's last menstrual period (LMP). It is the universal standard used in obstetrics and typically spans 40 weeks for a full-term pregnancy. It differs from fetal age, which counts from conception and is approximately 2 weeks less.
What is Naegele's Rule for calculating the due date?
Naegele's Rule is the standard obstetric formula for estimating the due date: add 280 days (or 9 calendar months plus 7 days) to the first day of the last menstrual period. It was formulated by German obstetrician Franz Karl Naegele in the early 19th century and remains the foundational method endorsed by ACOG for estimating EDD.
What if I don't know my last menstrual period date?
If the LMP is unknown or uncertain, a first-trimester ultrasound measuring the crown-rump length (CRL) between 8 and 13 weeks of gestation provides the most accurate gestational age estimate. ACOG recommends using this ultrasound date to establish or adjust the EDD when the LMP is unreliable or when there is a discrepancy of more than 7 days in the first trimester.
How accurate is gestational age calculated from LMP?
LMP-based gestational age is accurate to within approximately ±2 weeks for women with regular 28-day cycles. Its accuracy diminishes for women with irregular cycles, recent pregnancy loss, or recent use of hormonal contraceptives. First-trimester ultrasound is considered more accurate (±5–7 days) and is used clinically to confirm or revise LMP-based dating.
What are the three trimesters of pregnancy?
The first trimester spans weeks 0 through 12 and covers early organogenesis and the highest risk of miscarriage. The second trimester covers weeks 13 through 26, during which fetal growth accelerates and many structural anomalies are detectable via ultrasound. The third trimester runs from week 27 to delivery (typically by week 42), focusing on lung maturity, fetal positioning, and preparation for birth.
Last updated: 2025-01-15 · Formula verified against primary sources.