Transvaginal ultrasound (performed through the vagina) is safe during pregnancy for both mother and baby. There are no known risk factors associated with transvaginal ultrasound. This method is commonly used to evaluate fetal anatomy in the first trimester, cervical length, and placental location.
Transvaginal ultrasound may be offered for a number of reasons. The most common is to visualize and measure the maternal cervix. The cervix should be long and closed early in the pregnancy and this finding reduces the chance of a preterm delivery. Other times, transvaginal ultrasound may be indicated to assess the fetal structure since early in the pregnancy the fetus can be too small to adequately visualize through the maternal abdomen. Or later in pregnancy, fetal structures in the lower uterus can sometimes be best imaged by transvaginal scanning; for example, if the fetal head is down, this is an excellent way to image intracranial anatomy and brain structure or if the fetal bottom is down, then imaging of the lower spine and kidneys is enhanced. Occasionally, transvaginal ultrasound is used due to limitations experienced during the transabdominal ultrasound including fetal position, maternal body habitus, and overlying bowel gas that obscures the view of the sonographer.
If your doctor has a concern about the placental location and is worried placenta may be blocking delivery (complete placenta previa), transvaginal ultrasound is the best way to accurately assess the location of the placenta in relation to the cervix.