A dating scan is an early ultrasound examination performed during pregnancy. This scan uses sound waves to create images of the developing baby and provides initial insights into the pregnancy’s progression. It is typically one of the first imaging tests a pregnant person might undergo, offering foundational information about the gestational timeline.
Why and When a Dating Scan is Performed
A dating scan is recommended for several reasons, primarily to confirm pregnancy viability and establish the gestational age. It helps determine if the pregnancy is located correctly within the uterus, ruling out ectopic pregnancies. This early ultrasound can also identify whether the pregnancy involves a single baby or multiple babies, such as twins or triplets. Knowing the precise gestational age is important for scheduling subsequent prenatal tests at the appropriate times.
The optimal timeframe for performing a dating scan is typically between 8 and 14 weeks of gestation. However, many practitioners prefer the 7-12 week window because fetal development is uniform, allowing for highly accurate measurements. Performing the scan too early, before 7 weeks, can make it difficult to visualize the embryo or detect a heartbeat clearly. For individuals with irregular menstrual cycles or uncertainty about their last menstrual period, a dating scan is especially helpful in determining the pregnancy timeline.
The Scan Procedure
A dating scan can be performed either transabdominally (through the abdomen) or transvaginally (internally), depending on gestational age and the clarity of the images needed. For an abdominal scan, a clear gel is applied to the abdomen, and a transducer is gently moved across the skin to capture images. Patients are often asked to arrive with a full bladder for an abdominal scan, as this helps lift the uterus and provides clearer views of the pregnancy.
In very early pregnancy, especially before 8-9 weeks, a transvaginal ultrasound is often used for detailed images. This involves inserting a small, sterile-covered probe into the vagina, offering a closer view of the uterus and developing embryo. While this method may cause minor discomfort, it is not painful and is considered safe for both the pregnant person and the baby. The entire scan takes about 20 to 30 minutes.
What the Scan Reveals
The dating scan provides several key measurements and observations about the early pregnancy. One primary measurement is the Crown-Rump Length (CRL), the length of the embryo or fetus from the top of its head to the bottom of its torso. This measurement is the most accurate method for determining gestational age in the first trimester due to minimal biological variability in size among embryos at this stage. The sonographer carefully measures the CRL, excluding the yolk sac and limbs, to ensure precision.
Another important aspect the scan confirms is the presence of a fetal heartbeat. A heartbeat can be detected as early as 5-6 weeks of pregnancy, becoming clearer by 6-7 weeks. The detection of a heartbeat is an indicator of pregnancy viability. The scan also identifies the number of fetuses present. Based on the CRL measurement, the scan calculates an Estimated Due Date (EDD), which is considered more reliable than dates based solely on the last menstrual period.
Understanding Scan Accuracy
Dating scans, particularly those performed in the first trimester, are highly accurate for determining gestational age. The precision of dating by Crown-Rump Length (CRL) in the first trimester has a 95% confidence range of ±3 to 5 days. This accuracy is superior to dating based on the last menstrual period. If the estimated due date from a dating scan differs significantly from the last menstrual period calculation, the scan’s date is preferred due to its reliability.
The accuracy of the scan can be influenced by factors such as the timing of the scan; scans performed earlier in the optimal window tend to be more precise. While highly reliable, very early scans (before 7 weeks) may not show a clear embryo or heartbeat, potentially leading to a recommendation for a follow-up scan. Technical limitations or fetal positioning can also occasionally affect the clarity of the views. Overall, the dating scan provides a reliable estimate of gestational age, forming a foundation for subsequent prenatal care.