Pregnancy scans between 13 and 20 weeks

Posted on December 15, 2011

As a foetus continues to develop into the second trimester of pregnancy, an obstetrician is able to observe increasing amounts of information week by week than in early pregnancy scans. This is a critical part of the prenatal care process, and provides a great deal of information about the health of the foetus.

One of the earliest second trimester scans tends to be a nuchal scan. This is an ultrasound scan at between eleven and fourteen weeks of pregnancy to assess the amount of fluid around the neck of the foetus, high levels of which potentially indicating chromosomal abnormalities such as Down’s syndrome or Turner syndrome. With increased resolutions, a nuchal scan can be used to diagnose other less common chromosomal abnormalities as well as congenital heart defects. As ultrasound scanning is a non-invasive procedure, it does not pose a danger to either the mother or foetus. However, the procedure is only around 95% accurate, sometimes identifying false positives, a problem aggravated by operational error as performing the scan requires a high degree of skill. For this reason, a positive result generally has to be followed up with one or more invasive procedures to confirm or refute it.

An ultrasound scan at any time after around sixteen weeks can be used to determine the sex of the foetus, should the parents wish to know before delivery. This is normally included in the scan at twenty weeks which forms an major part of second trimester prenatal care. Obstetric ultrasonography at this stage can also check for multiple foetuses, miscarriage, and for other possible risks to the foetus and mother such as ectopic pregnancy, if this has not already been determined by an earlier scan, and the status of the umbilical cord, placenta and amniotic fluid. The scan will also check the status of the limbs and organs, in order to confirm that they are healthy or diagnose potential abnormalities, and determine the size of the foetus, normally a measure of the circumference of the abdomen. This is to check for intrauterine growth restriction, which can be caused by malnutrition of the foetus, and to closely monitor foetal growth in week by week scans. In addition to the more general checks, there are a number of specific malformations which can be diagnosed at this stage, such as hydrocephalus, cleft palate, dwarfism and club foot. A detailed ultrasound examination of the foetal spine can be used to check for spina bifida, but like the check for Down’s syndrome, an invasive procedure, normally amniocentesis, is required for confirmation, eliminating false positives.

This entry was posted in News, Obstetric

Tom Farrell

Written by Tom Farrell

Tom Farrell works in both the NHS and private sector providing gynaecology, pregnancy ultrasound and maternity care clinics in the Jessop Wing (NHS) and BMI Thornbury & Claremont Hospital (Private sector) in Sheffield.

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