A dating scan, often called an early ultrasound, is an imaging procedure performed in the first trimester of pregnancy to establish the Estimated Due Date (EDD) with a high degree of accuracy. The standard method for calculating a due date relies on the first day of the Last Menstrual Period (LMP), but this assumes a predictable 28-day cycle with ovulation occurring exactly two weeks later. When a scan suggests a significant difference, such as a four-week discrepancy from the LMP date, it can cause considerable concern for expectant parents. This large difference is unusual but is typically explained by biological factors related to conception timing.
How Dating Scans Measure Gestational Age
Dating scans measure the developing embryo or fetus, using size as a precise proxy for gestational age. The most reliable measurement in the first trimester is the Crown-Rump Length (CRL), which is the length from the top of the head to the bottom of the rump. During the first 12 weeks of pregnancy, the growth rate of all embryos is highly uniform, meaning a specific length consistently correlates to a specific gestational age.
The optimal window for dating is typically between 7 and 11 weeks. During this time, individual genetic or environmental factors have not yet caused variations in growth. After 12 weeks, the fetus’s growth becomes more individualized, and measurements like the Biparietal Diameter (BPD) and Head Circumference are used. These later measurements are less precise for determining age, and the accuracy of the ultrasound decreases significantly as the pregnancy progresses into the second and third trimesters.
Understanding the Margin of Error
For a first-trimester dating scan performed in the optimal window, the measurement is highly accurate, typically having a margin of error of only plus or minus five to seven days. This means that the true gestational age is likely within a week of the scan’s estimate. A four-week difference between the LMP date and the ultrasound date is therefore highly significant, falling far outside this expected margin of error.
If the scan was performed early, a four-week discrepancy does not suggest the scan is “wrong” in its measurement, but rather that the LMP date was inaccurate as a starting point. The ultrasound date, established by the physical size of the embryo, is generally considered the more reliable estimate of gestational age when such a large gap exists. The discrepancy points to a biological event that happened much later than the assumed ovulation date based on a standard menstrual cycle, such as delayed conception.
Key Reasons for Large Dating Discrepancies
The most frequent explanation for a four-week difference is late ovulation or irregular menstrual cycles. The LMP dating method assumes a 28-day cycle with conception occurring around day 14, but many women have longer or irregular cycles, which shifts the timing significantly. If ovulation and subsequent conception were delayed by three or four weeks, the embryo would naturally measure four weeks smaller than the date calculated from the last period. This scenario reflects a significant variation in the individual’s reproductive timing, and it does not imply a medical problem or issue with the pregnancy itself.
Other Biological Factors
Other biological factors can also contribute to a perceived lag in the dating. These include mistaking implantation bleeding for a light menstrual period, which causes the LMP date to be calculated incorrectly from the start. Technical measurement error, such as poor image quality or the sonographer measuring an abnormal structure, is a possible but less common reason for such a large discrepancy, especially in a high-quality clinical setting. In rare instances, a lag in ultrasound dating compared to LMP dating can also be an indication of early fetal growth restriction, although this is usually associated with a smaller, not a four-week, difference.
Clinical Follow-up After a Significant Date Change
When a dating scan reveals a difference of four weeks or more, healthcare providers will almost always use the ultrasound findings to establish a new Estimated Due Date. The first-trimester ultrasound measurement is recognized as the most accurate method for dating a pregnancy, overriding the less certain LMP calculation. This formal redating process ensures all subsequent pregnancy management aligns with the actual gestational age.
If the initial scan was performed very early, such as before six weeks, a follow-up ultrasound is often scheduled seven to ten days later. This repeat scan confirms viability and verifies that the embryo is growing at the expected rate, confirming the new due date is correct. Establishing an accurate date is crucial for scheduling time-sensitive prenatal screenings, determining the timing for labor induction, and accurately monitoring fetal growth throughout the pregnancy.