What is a mid-pregnancy anomaly scan?

A mid-pregnancy anomaly scan, also known as a morphological or 20 week ultrasound, together with the first scan or dating scan, is the most important and detailed ultrasound performed during a pregnancy. It is carried out between the 20th and 22nd week of the pregnancy.

What does it involve?

A mid-pregnancy anomaly scan is an external ultrasound (done on your tummy) very similar to a scan done during the third month of pregnancy. The scan consists in using a transducer, which has previously been covered in a gel that allows for better ultrasound waves transmission and for better image reception. The doctor is then able to show your baby’s face features and body more clearly.

What is the purpose of the mid-pregnancy anomaly scan?

A mid-pregnancy scan can be done between the 20th and 22nd week of pregnancy, in order to analyse the foetus’ anatomical (morphological) features. It is also done in order to verify whether or not there are any malformations. The scan analyses your baby’s biometrics (how much the foetus has grown), the position and structure of the placenta, how much amniotic fluid there is and the cervix’s general state of health.

This systematic morphological scan looks at:

• The central nervous system: thalami, ventricles of the brain, fornix, the corpus callosum, brain fissures or solci which can be seen at this stage of the pregnancy, cerebellum, cisterna magna and posterior cranial fossa;
• The baby’s face: nose and nostrils, lips, orbits, crystalline lenses, palate;
• The skeletal system: spinal column state of development, long bones, hand and feet;
• The chest: diaphragm, lungs, heart, great vessels;
• The gastrointestinal tract: stomach, liver, gallbladder, intestine;
• Genitourinary system: kidneys, bladder, genital organs.

When is the mid-pregnancy anomaly scan done?

This kind of scan must be done between the 20th and 22nd week of your pregnancy for two main reasons:

• It is the best moment for the foetus to be analysed due to an optimal ratio between the baby’s size and the amount of amniotic fluid;
• Abortion or termination of pregnancy is forbidden by law after 24 weeks of pregnancy.

If there is a high risk of malformation, it is necessary to carry out the scan between the 16th and 18th week (a genetic ultrasound or early mid-pregnancy anomaly scan) in order to carry out an early analysis of the baby’s growth progress. This is also helpful so that mum and dad can have a few specific genetic tests (such as foetal genome screening, array-CHG, cell-free foetal DNA test) in order to get a more accurate picture of their genetic situation.