When an ultrasound provides an estimated due date that does not align with your calculation, often based on your last menstrual period (LMP), it can cause confusion. This scenario is frequent in early pregnancy and is usually not a sign of a problem with the developing fetus. The discrepancy highlights the difference between two dating methods: one relies on a historical date, and the other uses a direct physical measurement of the embryo. Understanding how these dates are calculated provides clarity about your pregnancy timeline.
How Gestational Age Is Determined
The initial method for establishing a pregnancy timeline relies on the date of your last menstrual period. By convention, the estimated due date (EDD) is calculated by adding 280 days, or 40 weeks, to the first day of the LMP. This calculation is based on the assumption of a perfect 28-day menstrual cycle, with ovulation occurring precisely on day 14.
Conception itself happens about two weeks after the LMP, meaning this date includes a period when you were not actually pregnant. This method, while simple, is subject to inaccuracy because it relies on the consistency of the reproductive cycle.
The second, generally more accurate method involves using an early obstetric ultrasound to determine the gestational age (GA). In the first trimester, specifically between 6 and 13 weeks, the sonographer measures the Crown-Rump Length (CRL) of the embryo. The CRL is the measurement from the top of the head to the bottom of the torso, excluding the limbs and yolk sac.
This measurement is compared to a standardized chart of fetal development to estimate the age of the pregnancy. First-trimester ultrasound dating is considered the most precise method because all embryos tend to grow at a very uniform rate during this early stage.
Biological and Technical Causes for Mismatch
The primary reasons for a difference between the LMP date and the ultrasound date are rooted in biological variability. The LMP-based method assumes an ovulation date that may not reflect your actual cycle. Late or early ovulation, which can be affected by cycle length variations, stress, or hormonal changes, means that the actual date of conception may be several days off from the assumed date.
Even after fertilization, the timing of implantation can vary, taking anywhere from six to twelve days for the embryo to securely attach to the uterine wall. Since the LMP method cannot account for this range in periconceptional events, it introduces an inherent margin of error.
The ultrasound method, while highly accurate, also has technical limitations that contribute to minor discrepancies. A first-trimester ultrasound using the CRL has an accuracy of about plus or minus five to seven days. This means a one-week difference may simply fall within the expected range of measurement variability.
Later in pregnancy, the measurement accuracy decreases. Second-trimester scans are accurate within approximately ten days, and third-trimester scans can have a margin of error of up to three weeks. This decline occurs because, as the pregnancy progresses, genetic and environmental factors begin to cause individual variations in fetal growth rates.
When Doctors Adjust the Due Date
The medical team will assess the magnitude of the discrepancy to determine which date to use as the official estimated due date (EDD). Clinical guidelines recommend that if the ultrasound dating in the first trimester differs from the LMP dating by more than five to seven days, the EDD should be changed to align with the ultrasound measurement. The first-trimester ultrasound is prioritized because of its superior accuracy.
If the first ultrasound is not performed until the second trimester, the threshold for re-dating increases. For scans performed between 14 and 21 weeks, the due date is typically adjusted only if the difference exceeds ten days. This official date is then used consistently throughout the remainder of the pregnancy and is rarely changed again.
Accurate dating is important for the management of the pregnancy and the health of the fetus. The confirmed EDD provides a benchmark for monitoring fetal growth and identifying growth restrictions. It is also used to appropriately time prenatal screening tests and make decisions regarding the induction of labor or intervention for preterm delivery.