Dating scan

During the course of the pregnancy, every mother should have at least two scans. These help to ensure the pregnancy goes smoothly, and to identify any problems ahead of time. The dating scan is generally the first ultrasound that every mother-to-be will have.

What is the purpose of the dating scan?

While many women will have a good idea of when they had their last menstrual period, another portion will be far less certain. This can be due to a number of factors, including whether the pregnancy was planned or not, and if the mother often has irregular periods.

When you are offered a dating scan and the sonographer gains a picture of the baby’s development, they can use this to accurately calculate the baby’s age based on their size and formation. This allows more precise timing for later scans, doctors’ appointments and check-ups on both mother and baby. As medical professionals have a precise idea of the baby’s development over nine months, knowing the foetal age makes it easier to judge if everything is fine with the baby. The scan also gives a due date, which is sometimes a small range of dates due to a difference with your own dating attempt. An accurate due date is crucial so that, in the event of being overdue, your doctor knows whether you are drastically overdue or simply a few days late.

When does the dating scan take place?

As the scan is designed to assist the doctors’ understanding of the foetal age and when the baby is due, determining when this early scan takes place is occasionally a matter of estimation. Your doctor will ask you questions about when you had your last menstrual period, and estimate that ovulation occurred 14 days into the cycle, but some mothers may be unsure. However this is not usually a problem, as the best time for a dating scan falls between 8 and 12 weeks, a month-long period. Dating scans are much less reliable after 13 weeks because at this stage babies often start growing at different rates.

What does the sonographer look for?

The sonographer (the qualified professional who performs the scan) looks for key measurements to find out your baby’s age. These are basic calculations such as the length from crown to rump (head to the end of the bottom), the femur length (longest bone in the body), and the head circumference. Some of these and other measurements are most useful after 13 weeks, which is beneficial for a late dating scan.

The baby’s health

In addition to foetal measurements, this initial scan provides an opportunity to assess the baby’s health. For these purposes, the sonographer looks for the foetal heartbeat and any currently visible abnormalities. If the heartbeat cannot be detected then more tests have to be done, but this does not necessarily indicate a problem: the baby’s position and other factors can obscure the results of an ultrasound. An experienced sonographer can often find any major irregularities like limb deformities.

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