Article — Gestational Age Calculator
Gestational Age Calculator: LMP, Conception, and Ultrasound Methods
Gestational age (GA) is the time since the first day of the last menstrual period, written in weeks and days, for example 28+3. The calculator on this page offers three methods: LMP, conception date, and first-trimester ultrasound crown-rump length. The estimated due date (EDD) is LMP plus 280 days, the rule Franz Naegele described in 1812 and ACOG still endorses.
For most pregnancies the LMP method and a first-trimester ultrasound agree within a week. When they differ by more than 5 to 7 days before 14 weeks, ACOG Committee Opinion 700 says use the ultrasound date. After 14 weeks, the LMP-based date holds unless ultrasound disagreement is large.
What is gestational age?
Gestational age counts from the first day of the last period, not from conception. The naming is historical: women know when their period started; almost no one knows the exact moment of ovulation. By convention, the two-week lag from LMP to fertilization is baked into every clinical formula and every chart of fetal milestones.
Fetal age, sometimes called conceptional age, runs about 14 days behind gestational age. A 12-week gestational ultrasound shows a 10-week-old embryo. Obstetricians, midwives, and dating tables all speak in gestational age, so that is the number to track.
Term pregnancy is now defined as 39 weeks 0 days through 40 weeks 6 days. Births at 37 to 38 weeks are early term, and 41 weeks is late term. The change came from a 2013 ACOG and SMFM consensus that recognized higher neonatal risk at earlier weeks.
Gestational age from LMP
The LMP method is the simplest formula. Subtract the date of the first day of the last menstrual period from today, divide by seven for weeks, and the remainder gives extra days. The calculator does this in one click. Accuracy depends on knowing the LMP and on cycle regularity.
For a 28-day cycle, ovulation falls about day 14 and conception occurs around day 14 to 15. For a longer cycle, say 35 days, ovulation drifts to day 21. The LMP-based GA over-estimates true age unless the formula shifts the LMP forward by the cycle difference. The calculator does this automatically when you change the cycle length input.
LMP method GA(days) = today - LMPConception GA(days) = today - conception + 14CRL (Robinson) GA(days) = 8.052 · √CRL(mm) + 23.73EDD (Naegele) LMP + 280 daysGestational age from ultrasound
Crown-rump length (CRL) is the straight-line measurement from the top of the embryo's head to the bottom of its torso. Between roughly 7 and 13 weeks, embryo size grows in a tight, predictable curve, making CRL the best single dating measurement we have. The Robinson-Fleming 1975 regression converts millimeters of CRL into days of gestation: GA = 8.052 times the square root of CRL in millimeters plus 23.73.
After 13 weeks the embryo elongates and curves; biparietal diameter, head circumference, and femur length take over as dating biometrics. By the third trimester, growth is too variable for precise dating. A first-trimester scan dates a pregnancy within 3 to 5 days; a third-trimester scan only manages 14 to 21 days.
How to use the gestational age calculator
Pick the method that matches your data. LMP works if you remember the date and your cycles run regularly. Conception works if you tracked ovulation or know the precise date of insemination. CRL works if you have an ultrasound report with the embryo measurement in millimeters and the date of the scan.
The widget returns weeks plus days, the trimester, the estimated due date, and the count of days remaining until that date. All three methods produce results in the same format, so flipping between them is a quick way to cross-check dating.
If your LMP and ultrasound dates differ by more than a week in the first trimester, write down both, then ask your obstetric provider which date will be used for the prenatal schedule. The pregnancy then runs on one number, not two.
Gestational age and trimesters
Trimesters are administrative buckets that group milestones. The first trimester runs from week 0 through week 13, covering organogenesis, the nuchal translucency window at 11 to 14 weeks, and most miscarriages. The second trimester spans weeks 14 to 27, anchored by the anatomy scan at 18 to 22 weeks and the first quickening, usually between 18 and 25 weeks.
The third trimester starts at 28 weeks. Growth scans, glucose tolerance testing, and group B strep screening at 36 weeks all sit in this window. Babies born at 24 weeks now survive with intensive care, but viability is a sliding line and is not the same as healthy term delivery.
- Early term 37+0 to 38+6 weeks
- Full term 39+0 to 40+6 weeks
- Late term 41+0 to 41+6 weeks
- Post-term 42+0 and beyond
- Preterm earlier than 37+0 weeks
- Extremely preterm earlier than 28+0 weeks
Gestational age and the due date
The estimated due date is the day a pregnancy reaches 40 weeks 0 days, exactly 280 days from LMP. Only about 5 percent of births occur on the EDD, and roughly two thirds happen within 14 days either side. The EDD is a planning anchor, not a deadline.
Once dating is fixed by an early scan, the EDD usually does not move. Late-trimester scans that suggest a different age are interpreted as growth variation, not new dating evidence. This convention prevents needlessly induced labor based on noisy late measurements.
This calculator is for information. Real prenatal care requires a clinician who can interpret dating in the context of cycle history, prior pregnancies, ultrasound quality, and any abnormal findings.
Common gestational age mistakes
The first mistake is using the conception date with the LMP formula. The calculator handles this if you pick the conception mode, but a hand calculation will be 14 days off if you do not add the offset. The second is ignoring irregular cycles. A woman with a 35-day cycle who uses the 28-day default will be told she is a week further along than she really is.
The third is over-trusting late-trimester ultrasound dating. By 30 weeks the same fetus could be measured plus or minus 21 days from true age depending on growth pattern. If first-trimester dating exists, it almost always wins.