The uterus is a large muscle made up of thousands of muscle cells. When calcium enters these cells, the muscle contracts and tightens. When calcium flows back out of the cell, the muscle relaxes. Calcium channel blockers work by preventing calcium from moving into the muscle cells of the uterus, making it less able to contract.
Nifedipine can reduce the number and frequency of contractions, though, this effect and how long it lasts varies from woman to woman. Like all tocolytic medications, however, calcium channel blockers do not consistently prevent or delay preterm delivery for a significant period of time.
Even so, studies have shown that calcium channel blockers can delay delivery for at least several days (depending, of course, on how far dilated a woman's cervix is when the medication is started.) This isn't a lot of time, but it can make a big difference for the fetus if the mother is given steroids along with calcium channel blockers. After 48 hours, steroids improve a baby's lung function and reduce his or her risk of dying by 40%. For more information on steroid therapy, go to Corticosteroids
For the Mother
The most common side effects in the mother include headache, nausea, and flushing. Nifedipine can also cause a woman's blood pressure to drop.
For the Baby
Nifedipine causes few side effects in the baby. However, if the mother's blood pressures drops for a prolonged period of time, it can affect blood flow to the baby.
Women with medical conditions that could be made worse by the side effects described above should not be given calcium channel blockers. This includes women with low blood pressure, heart failure, or disorders that affect muscle strength.