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Pregnancy Bliss | Reproductive Health Hub

5. Ultrasound scan at 18 - 20 weeks

In the UK, as in many industrialised countries, the majority of hospitals offer a routine mid-trimester ultrasound scan to all pregnant women. It is normally termed a "detailed anatomy scan", because this is the principal purpose of the scan:: to do a detailed surveillance of the anatomy of the fetus, with the aim of detecting any major anatomical anomalies.
Is this routine scan beneficial or even desirable? This is not as easy to answer as one may be tempted to believe. Nonetheless, there is strong evidence that most mothers desire it and actually look forward to it.

The fact that this examination is widely termed an "anatomy scan" does create a few problems, however. It may create an impression in the parents' minds that their baby is definitely getting an absolute all-clear at this stage. When, in the end, this turns out not to be the case, this innocent fact may unleash anxiety and a suspicion that something was held back from them. Unfounded as this might be, it can and has occasionally caused considerable distress to some parents.
It is therefore important that all obstetric units offering this service should ensure that pre-scan information and counselling is not cursory but detailed, so that every single woman has realistic expectations.

The potential benefits and limitations of this examination are spelt out in detail in this chapter. Most obstetric units have a set-up which ensures the comfort and interaction of the parents-to-be in the examination using highly skilled technicians who are able to explain things as the examination is in progress. It is this element which most parents appreciate the most. In effect, it is the first virtual bonding session between the unborn baby and his or her parents.
In this section we explain what the detailed mid-trimester (eighteen to twenty weeks) scan entails.

The benefits of pregnancy ultrasound scanning

Is an ultrasound scan necessary for every pregnant woman at this gestation?
No. It is not necessary. It is, however, almost universally accepted that this is an extremely useful measure which benefits the expectant mother, her family, the care-givers and in some cases, the unborn baby. It is important to stress that, for most, everything will be found to be normal.

What are the potential benefits of a twenty weeks ultrasound scan?

It confirms the number of fetuses. Many twin pregnancies are established for the first time at this scan.

It confirms viability. In a few cases, the fetus may die in the womb and the expectant mother may get no warning whatsoever. This is called "missed abortion". An ultrasound scan arranged routinely at this stage may be the first opportunity to discover this unfortunate turn of events. This is an uncommon finding.

It confirms gestation. Very often, an ultrasound scan may show that the pregnancy is far more or less advanced than suggested by the mother's dates. This has important implications, as we shall see shortly.

It identifies high-risk pregnancies. Potential problems may be identified for the first time at the twenty-weeks ultrasound scan. This will allow for the necessary input at the appropriate time.

It identifies gross anomalies.This scan is used to do a detailed anatomy assessment. Most major malformations will be identified. This allows for proper analysis and action. Occasionally, fetal malformations where the baby has no chance of surviving outside the womb are discovered. This enables the mother, her family and the caregivers to reach a decision on what needs to be done. Some non-lethal problems, which can be treated either in the womb or after delivery, may also be identified.
Early trimester Placenta and fluid 20 weeks scan Safety Uses in pregnancy Limitations 4D scans Fetal gender