Gestational Age Calculator
Calculate gestational age in weeks and days from the last menstrual period or ultrasound.
Defaults to today
Defaults to today
Gestational Age
Estimated Due Date
Trimester
Days to Due Date
Pregnancy Progress
Understanding Gestational Age
Gestational age is the length of a pregnancy measured from the first day of the woman's last menstrual period (LMP), not from the date of conception. Because conception typically occurs about two weeks after the start of the last period, gestational age is roughly two weeks greater than the actual age of the embryo or fetus (the "fertilization age"). Gestational age is expressed in completed weeks and days, for example "12 weeks and 3 days" (written 12w3d), and a full-term pregnancy is considered to be 40 weeks (280 days) from the LMP.
The Three Trimesters
| Trimester | Gestational Age | What Happens |
|---|---|---|
| First | 0w0d - 13w6d | Organ formation and rapid early development |
| Second | 14w0d - 27w6d | Growth, movement, and anatomy scan |
| Third | 28w0d - 40w+ | Final maturation and weight gain before birth |
Dating Methods Explained
Last Menstrual Period (LMP) Dating
LMP dating, also known as Naegele's rule, estimates gestational age by counting forward from the first day of the last menstrual period. It assumes a regular 28-day cycle with ovulation on day 14. The current gestational age is simply the number of days between the LMP and the "as of" date, and the estimated due date (EDD) is the LMP plus 280 days.
Gestational Age = As Of Date − LMP
EDD = LMP + 280 days
LMP dating is convenient but can be inaccurate if cycles are irregular, ovulation is late or early, or the exact date of the last period is uncertain.
Ultrasound Dating
Ultrasound dating uses measurements of the embryo or fetus to assign a gestational age. You enter the gestational age the sonographer measured on the scan date (in weeks and days), and the calculator carries that age forward to the "as of" date. The estimated due date is anchored to the scan: it is the scan date plus the number of days still remaining until 280 days of gestation.
Current GA = Scan GA + (As Of Date − Scan Date)
EDD = Scan Date + (280 − Scan GA in days)
LMP vs Ultrasound: Which Is More Accurate?
Ultrasound dating is generally more accurate than LMP dating, especially when performed early in pregnancy. In the first trimester, the crown-rump length (CRL) - the measurement from the top of the head to the bottom of the buttocks - gives the most reliable estimate, accurate to within about 5 to 7 days. As pregnancy advances, biological variation in fetal size grows, so later ultrasounds are less precise: a second-trimester scan is accurate to roughly 7 to 14 days and a third-trimester scan to only 14 to 21 days.
LMP dating depends on the woman remembering the exact start of her last period and on having a predictable cycle. Irregular cycles, late or early ovulation, breastfeeding, recent hormonal contraception, and early bleeding that is mistaken for a period can all throw off the estimate. For these reasons, clinical guidelines recommend using an early ultrasound to confirm or revise the due date whenever the ultrasound estimate differs from the LMP estimate by more than a set number of days (commonly 5 to 7 days in the first trimester).
In practice, when a reliable first-trimester ultrasound is available, it is usually used to set the official gestational age and due date, while the LMP serves as a useful cross-check.
References
The dating rules and trimester definitions used in this calculator are based on established obstetric guidelines:
- American College of Obstetricians and Gynecologists (ACOG). "Methods for Estimating the Due Date." Committee Opinion No. 700. ACOG Committee Opinion 700
- Naegele, F.K. "Naegele's rule for estimated date of delivery." Standard obstetric dating method (LMP + 280 days).
- World Health Organization (WHO). "WHO recommendations on antenatal care for a positive pregnancy experience." WHO Antenatal Care Guidelines
Note: This calculator provides estimates based on standard obstetric dating rules and is intended for general information only. Gestational age and due dates are estimates - only about 4% of babies are born on their exact due date. The figures here do not replace the assessment of a qualified healthcare provider. Always rely on the dating provided by your doctor or midwife, who can account for your individual cycle, ultrasound findings, and medical history.
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