Second Trimester Ultrasound
Obstetric Ultrasound for Anatomy Survey
The second trimester ultrasound is typically done between 18 and 22 weeks of pregnancy. It is usually the first time parents see their baby. Keepsake photos are often taken – some are two-dimensional (2D) and some may be three-dimensional (3D). This is also the time when parents may be able to find out the sex of their baby. Second trimester ultrasound serves a much greater purpose, however. It is used to survey the anatomy of the fetus to determine if everything is developing correctly.
Preparation
Preparation for the second trimester ultrasound is a bladder prep. A full bladder is necessary to clearly see the cervix. It also pushes the fetus up so it can be more fully examined. Ultrasound cannot be used to see through bone. If the fetus is low in the mother's pelvis, it can be hidden by the pelvic bones.
Uterus and Adnexa
The uterus and adnexa are examined. The adnexa are the areas to each side of the uterus. They contain the ovaries. The placenta is examined and its location noted. The cervix, the lower portion of the uterus that remains closed for the duration of the pregnancy, is also examined. Transvaginal ultrasound may be used to clearly visualize the cervix. The cord insertion into the placenta may be noted. An amniotic fluid index (AFI) is taken to determine how much amniotic fluid is present.
Fetal Growth
Several measurements of the fetus are taken to determine an approximate size and weight. These measurements are used to assess whether the fetus is growing at an appropriate rate since the age and due date are usually already determined by last menstrual period dating or first trimester ultrasound. These measurements include:
- Head circumference
- Biparietal diameter – the measurement from one side of the head to the other
- Abdominal circumference
- Femur length – the length of the upper leg bone; the bones of the lower legs and arms are imaged
Head
A survey of the head anatomy is a strong indicator of whether a genetic anomaly or other problem may be present. The brain, face, and head shape often show several signs for several trisomies, including Down Syndrome (trisomy 21). Surveyed head anatomy may include:
- Choroid plexus
- Lateral ventricles
- Cerebellum
- Cisterna magna
- Nuchal fold (fold of skin on the back of the neck)
- Cavum septum pellucidum (CSP)
- Orbits (eye sockets)
- Lips and nose
- Profile, including the nasal bone
Spine
The spine is imaged in two planes: lengthwise (long) and crosswise (transverse). The abnormality most often visualized is spina bifida, the incomplete closure of the spine and its contents. The long view of the spine is typically acquired in one image. The transverse images are acquired at the level of the neck or clavicles (cervical), heart (thoracic), kidneys (lumbar), and iliac bones (sacrum).
Chest and Abdomen
The chest and abdomen are examined for many possible anomalies. Surveyed anatomy may include:
- Four-chamber heart
- Left ventricular outflow tract (aorta leaving the heart)
- Right ventricular outflow tract (pulmonary arteries leading to the lungs)
- Diaphragm
- Stomach
- Kidneys
- Bladder
- Cord insert – where the umbilical cord attaches to the abdomen.
3D and 4D Prenatal Ultrasound
Three-dimensional (3D) ultrasound is a reconstruction of the fetus. Four-dimensional (4D) ultrasound is that reconstruction in real-time, so the "fourth dimension" is time. This means that a 3D image is seen while the baby is moving. These images can be quite difficult to acquire. There must be amniotic fluid around the part of the baby that is being imaged. Unfortunately, many babies keep their arms in front of their faces, or they rest their faces against the placenta or uterine wall.
Ultrasound Machine
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