Many medical personnel suggest having an ultrasound around 20 weeks of pregnancy to check for possible physical problems with the baby, or the placenta or fluid inside the uterus. While you can refuse a scan around 20 weeks, most expectant parents are anxious to see their baby on screen and have the baby checked out for problems. Many fetal complications, but not all, can be seen on an ultrasound at 20 weeks, according to BabyCentre.co.uk.
Starting at the head and working down, the ultrasound technician checks each part of the baby for problems. Complications that can be identified include anencephaly, a missing portion of the brain, cleft lip or other facial abnormalities, limb deformities, heart anomalies and spinal deformities. The stomach, intestine and kidneys are examined for malformations, states BabyCentre.co.uk. At this point, most babies can be identified as male or female, unless they refuse to open their legs or turn so that reproductive organs can be seen.
To ensure that the baby is growing properly, certain bones are measured to assess fetal growth. The head circumference and diameter, plus the abdominal circumference and the length of the longest bone in the body, the femur, are measured and compared to standard growth charts, states BabyCentre.co.uk.
The location of the placenta is noted; at the midpoint of pregnancy, the placenta is still often implanted fairly low on the uterine wall. If the placenta is noted to be lower than normal, a follow-up scan is done to make sure the placenta migrates up the slope of the uterus and away from the opening to the uterus, the cervix, as the uterus grows. A low-lying placenta, or placenta previa, which covers the cervix, can cause serious bleeding later in pregnancy and necessitates cesarean delivery.
The amniotic fluid around the baby is also measured; too much fluid, called polyhydramnios, or too little fluid, called oligohydramnios, can indicate a problem with fetal kidneys or intestines, states Ob-Ultrasound.net. The cervical length may also be measured, especially if there's a previous history of preterm delivery.
Certain physical signs on the baby are considered to be soft markers, or indications that the baby may have a chromosomal abnormality such as Down syndrome. These abnormalities can often be seen on an ultrasound, and help assess the risk for chromosomal abnormalities. However, they are not diagnostic in most cases and may be found in normal fetuses as well, reports EarlyRiskAssessment.com. Some of the common markers for Down syndrome include a nuchal (neck) skin fold thickness of more than 6 mm, heart defects, shortened femur and arm bones, intestinal abnormalities and the presence of a choroid plexus cyst, a cyst that forms in an area of the brain, the choroid plexus, that makes fluid that surrounds the spinal cord and brain, according to Ob-Ultrasound.net.