At 18 weeks, your baby-to-be is large enough that your sonographer will perform the ultrasound transabdominally. With a transabdominal exam, the sonographer will first place gel on your abdomen so that the transducer can slide more easily over your tummy. The transducer is a radio soundwave-emitting device that helps produce an image of your baby in utero that the sonographer then uses to evaluate your baby's health.
Placenta previa: Implantation of the placenta in the lower portion of the uterus, covering part of the cervix. Placenta previa can result in bleeding during pregnancy and could affect the method of delivering the baby.
Neural tube defects (NTD): Abnormalities of the spine, such as spina bifida, and related abnormalities of the fetal brain.
Images courtesy of the American Institute of Ultrasound in Medicine (AIUM.org).