What is an ultrasound?
In prenatal testing, sound waves pass through the mother's abdomen to the fetus and then bounce back and create the fetal image on an electronic monitor. Think of it as high-frequency sound bouncing off of human tissue. When the sound reflects from tissues, it is read as darker or lighter, depending on the density of the tissue it is scanning. Images will appear on a video screen as a series of dots, with denser tissue, such as bones, showing up as white, softer tissues appearing darker and fluids appearing as black.

What types of ultrasound are used during pregnancy?
There are two approaches used during pregnancy -- transabdominal (through the abdomen) and transvaginal (via the vagina). The transabdominal ultrasound can be used throughout pregnancy but most commonly is used in the second and third trimesters. It provides a clear view of the fetus and the placenta. The transvaginal approach is used in the first trimester of pregnancy to visualize the cervix, the uterus, gestational sac, embryo and other deep pelvic structures.

What does each procedure entail?
When using the transabdominal approach, the woman must have a full bladder. She should drink lots of water (approximately one to two quarts) one hour prior to the procedure. This will help to move the intestines out of the way and aid in distinguishing various fetal parts. For the abdominal ultrasound, a lubricating gel is applied to the abdomen. A probe is then placed over the gel and moved all around on the abdomen. This procedure takes approximately 10 to 30 minutes. The technician can video a moving image or freeze a picture for permanent records.

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