Gestational age is most accurately determined by a combination of your last menstrual period and an early ultrasound, ideally performed before 14 weeks. When both methods are available, the ultrasound measurement is accurate to within 5 to 7 days and serves as the gold standard for dating a pregnancy. Here’s how each method works and when it matters most.
Dating From Your Last Menstrual Period
The simplest starting point is your last menstrual period (LMP). A formula known as Naegele’s rule adds nine months and seven days to the first day of your last period to estimate your due date. This calculation assumes a 28-day cycle with ovulation occurring on day 14, which means gestational age technically begins about two weeks before conception actually happens.
The method works well for people with regular, predictable cycles. But many factors throw it off: cycle lengths that differ from 28 days, irregular ovulation, recent use of hormonal birth control, or simply not remembering the exact start date. In one study of women with irregular periods, the LMP-based estimate differed from ultrasound dating by more than a week in over half of cases and by more than two weeks in about a quarter. Among those women, the rate of pregnancies classified as “post-term” dropped from 20% using LMP dating down to just 2.5% when ultrasound was used instead. That gap matters because post-term classification can trigger medical interventions that may not be necessary if the dates are simply wrong.
Early Ultrasound: The Most Reliable Method
Ultrasound in the first trimester is the most accurate way to pin down gestational age. Before an embryo is even visible, typically around 5 to 6 weeks, a provider can measure the gestational sac itself. The sac is measured from inside wall to inside wall in multiple directions, and the average of those measurements is compared against reference tables or calculated by the ultrasound machine’s built-in software.
Once an embryo becomes visible, usually around 7 weeks, the key measurement shifts to crown-rump length: the distance from the top of the head to the bottom of the torso. According to guidelines from the American College of Obstetricians and Gynecologists (ACOG), crown-rump length measured up to 13 weeks and 6 days is accurate to within 5 to 7 days. That narrow margin makes first-trimester ultrasound the benchmark against which all other dating methods are compared. When ultrasound dating and LMP dating disagree by more than a certain threshold, the ultrasound date typically takes priority.
Second and Third Trimester Measurements
After about 12 weeks, individual embryos start varying more in size, and a single head-to-rump measurement no longer captures growth well. Providers switch to a set of four measurements collectively known as fetal biometry:
- Head diameter (biparietal diameter): measured across the widest part of the skull
- Head circumference: the distance around the skull
- Abdominal circumference: the distance around the belly
- Thighbone length (femur length): the length of the upper leg bone
These measurements are plugged into formulas that estimate both gestational age and fetal weight. They’re still useful in the second trimester, but accuracy decreases as pregnancy progresses. By the third trimester, fetal biometry becomes considerably less reliable for dating because babies of the same gestational age can differ significantly in size based on genetics, nutrition, and other factors. A measurement that’s accurate to about a week at 14 weeks could be off by two to three weeks at 30 weeks. This is why providers place so much emphasis on getting that early ultrasound right.
Fundal Height: A Quick Physical Check
Starting around 24 weeks, your provider will likely measure fundal height at routine visits. This is the distance in centimeters from your pubic bone to the top of your uterus, measured with a tape measure. After 24 weeks, that number in centimeters roughly matches your gestational age in weeks, give or take about 3 centimeters. So at 30 weeks, you’d expect a measurement somewhere between 27 and 33 centimeters.
Fundal height isn’t precise enough to establish a due date on its own. Its real value is as a screening tool: if the measurement is significantly larger or smaller than expected, it can flag potential issues like excess amniotic fluid, growth restriction, or an incorrect due date that needs a second look with ultrasound.
Dating for IVF and Assisted Reproduction
If you conceived through IVF, gestational age is calculated differently because the exact date of embryo transfer is known. For a day-three embryo transfer, your due date is 263 days from the transfer date. For a day-five embryo transfer, it’s 261 days. These calculations remove the guesswork around ovulation timing entirely, making IVF pregnancies some of the most precisely dated.
Why hCG Levels Don’t Work for Dating
Blood levels of the pregnancy hormone hCG rise rapidly in early pregnancy, and you might wonder whether those numbers can tell you how far along you are. In practice, they can’t. The normal range at any given week is enormous. At 12 weeks, for example, the median hCG level is around 56,000, but the normal range spans from roughly 23,000 to 115,000. Two women at the exact same gestational age could have hCG levels that differ by fivefold and both be perfectly normal. Serial hCG tests are valuable for confirming a pregnancy is progressing in very early weeks, but the overlap between weeks makes them unreliable for pinpointing gestational age.
When Dates Conflict
It’s common for LMP dating and ultrasound dating to produce slightly different results. When the two estimates are close, providers generally stick with the LMP-based date. When they diverge beyond a certain threshold, the ultrasound date wins. The practical impact of accurate dating goes beyond curiosity about your due date. It affects the timing of screening tests, decisions about preterm versus term labor, and whether a pregnancy is classified as post-term. Getting it right early, ideally with a first-trimester ultrasound, gives the clearest picture for the rest of pregnancy.