Patients may visit their physician weekly during the third trimester of pregnancy (week 25 to delivery). The physician will continue to monitor the patient’s weight and blood pressure as well as fetal movement and activity during this time. Third-trimester prenatal care visits also include:

• GBS (group B streptococcus) testing. Patients will be screened for GBS, a typically harmless bacterium that may inhabit the body, at some point between 33 and 36 weeks of pregnancy. The physician will take a culture from just inside the woman’s vagina and rectum. The culture will then be tested for the bacterium. GBS poses no health risks to expectant mothers. However, it can be passed to a baby during labor and delivery and this infection can be serious to the newborn resulting in blindness, deafness, mental retardation, physical disability and even death. If this bacterium is detected, the patient will be given antibiotics when she begins labor.
  
  
• Checking the position of the baby (presentation of the fetus). The physician may be able to determine the baby’s position during the patient’s seventh month of pregnancy. Most babies move into a head-down position in the uterus a few weeks before they are born. The physician will check the baby’s presenting part, the portion of the baby that is furthest down in the pelvis, at this time. The physician can often feel the baby’s head in the lower abdomen just above the patient’s pubic bone, or at the top of her birth canal, during a vaginal examination. An ultrasound may be performed if the presenting part cannot be determined with a vaginal exam.
  
  Babies positioned rump- or feet-first are in “breech position,” and must usually be delivered by Caesarean section. If the baby is breech, and is not positioned too far down in the patient’s pelvis, the physician may perform an external version approximately two to four weeks before the baby is due. This procedure involves applying pressure to the abdomen in an effort to move the baby into the desirable head-down position. The physician may determine the “station” of the baby’s presenting part, or how far down the baby is positioned in the pelvis, closer to the baby’s due date.
  
  
• Checking to see if the patient’s cervix is dilated and effaced. As the baby’s due date grows nearer, a vaginal exam will determine the following:
  
  
• How much the cervix is starting to soften
• How much the cervix has dilated (opened)
• How much the cervix has effaced (thinned out)
  
  
The progress of a patient’s dilation and effacement is measured in centimeters (cm), and percentages (e.g., 3 cm dilated and 30 percent effaced). Babies are not delivered until the cervix is 10 cm dilated and 100 percent effaced.

Patients should not put too much emphasis on these measurements. Generally, this exam is used only to predict if the patient is a good candidate for induction of labor. Some women may be 3 cm dilated for weeks, whereas others may go into immediate labor that is not preceded by any dilation or effacement. In fact, the physician may not even perform this exam unless the patient is being considered for labor induction.