Calculating the gestational age (GA) is a fundamental part of prenatal care. GA is measured in weeks and days and establishes the Estimated Due Date (EDD). An accurate due date calculation is important because it dictates the proper timing for vital prenatal screenings, diagnostic tests, and interventions, such as monitoring fetal growth and planning for a safe delivery. The healthcare team relies on this initial dating to assess whether the fetus is growing as expected and to determine if the pregnancy is proceeding normally. Without a reliable GA, managing the pregnancy becomes significantly more challenging.
Using Timing and Dates
The initial estimation of gestational age typically begins with the first day of the last menstrual period (LMP) provided by the patient. This method uses Naegele’s Rule, which assumes an average 28-day menstrual cycle with conception occurring approximately two weeks after the LMP. The rule calculates the EDD by adding seven days to the first day of the LMP, subtracting three months, and then adding one year, resulting in a median pregnancy duration of 40 weeks.
This method provides a quick starting point, but its accuracy depends entirely on the patient’s memory and the regularity of their menstrual cycle, making it subject to error. Ovulation timing can vary greatly, and irregular cycles make the assumption of a 28-day cycle unreliable. When the exact date of conception is known, such as through assisted reproductive technologies like in vitro fertilization (IVF), the dating becomes much more precise. The date of embryo transfer or egg collection is used as the most accurate reference point for determining gestational age.
Physical Examination Methods
Healthcare providers may use physical examination techniques to assess the size of the uterus, which offers a supportive estimate of gestational age. In the first trimester, a pelvic examination can manually assess the size of the uterus, correlating roughly with the expected size for the early weeks of gestation. This early assessment helps confirm the pregnancy is progressing but is not precise enough to set a definitive due date.
A more common physical method used later in pregnancy is measuring the fundal height, which is the distance from the top of the pubic bone to the top of the uterus. This measurement is taken in centimeters with a tape measure. After about 20 weeks of gestation, the fundal height in centimeters should correspond closely to the number of weeks the patient is pregnant, typically within two to three centimeters. The measurement is primarily used to track fetal growth over time, not to establish the initial due date.
Physical measurements have significant limitations and are not a substitute for imaging. Factors such as the patient’s body size, the position of the fetus, the amount of amniotic fluid, or carrying multiples can all affect the fundal height measurement. A measurement that is larger or smaller than expected often prompts the provider to order an ultrasound for a more accurate evaluation of fetal growth and size.
Ultrasound Confirmation and Dating
Ultrasound imaging is the most accurate method for establishing or confirming gestational age and the estimated due date, especially when performed early in the pregnancy. The accuracy of ultrasound dating decreases significantly as the pregnancy progresses because fetuses exhibit more natural biological variation in size later in gestation. The first trimester is considered the gold standard window for dating, offering precision within approximately five to seven days.
First Trimester Dating
During this early stage, the primary measurement used is the Crown-Rump Length (CRL), which is the length of the embryo or fetus from the top of its head to its rump. Since all embryos grow at a consistent rate in the first trimester, the CRL measurement, typically taken between 8 and 13 weeks, provides the most reliable estimate of gestational age. This measurement is so accurate that it supersedes a due date calculated by the LMP.
Second and Third Trimester Measurements
In the second and third trimesters, a variety of biometric measurements are taken to estimate gestational age, though these are less accurate for dating than the first-trimester CRL. These measurements include the Biparietal Diameter (BPD), which is the width of the fetal head; the Head Circumference (HC); and the Femur Length (FL), which is the length of the thigh bone. Measurements are often combined into a composite to provide a better estimate, accurate to within about 10 days in the second trimester.
The later measurements are primarily used for tracking the fetus’s growth and development against the established due date. By the third trimester, the accuracy for dating drops to an error range of three to four weeks, meaning any initial due date established by an early ultrasound is rarely changed after the first trimester.