Maternal Changes Before the onset of Labor

MATERNAL CHANGES BEFORE THE ONSET OF LABOR

As patients approach term, they experience uterine con-tractions of increasing strength and frequency. Sponta-neous uterine contractions, which are not felt by the patient, occur throughout pregnancy. Late in pregnancy they become stronger and more frequent, resulting in the patient’s perception of discomfort. TheseBraxton Hickscontractions (false labor) are not associated with dilation of thecervix, however, and do not fit the definition of labor. It is fre-quently difficult for the patient to distinguish these often uncomfortable contractions from those of true labor. As a result, it is difficult for the physician to determine the true onset of labor by history alone. Braxton Hicks contrac-tions are typically shorter in duration and less intense than true labor contractions, with the discomfort being charac-terized as over the lower abdomen and groin areas. It is not uncommon for these contractions to resolve with ambulation, hydration, or analgesia.

True labor is associated with contractions that the patient feels over the uterine fundus, with radiation of discomfort to the low back and lower abdomen.

These contractions become increasingly intense and frequent.

Another event of late pregnancy is termed “lighten-ing,” in which the patient reports a change in the shape ofher abdomen and the sensation that the baby is lighter, the result of the fetal head descending into the pelvis. The patient may also report that the baby is “dropping.” The patient often notices that her lower abdomen is more promi-nent, and she may feel a need to urinate more frequently as the bladder is compressed by the fetal head. The patient may also notice that she is breathing more easily, because there is less pressure on the diaphragm as the uterus becomes smaller.

Patients often report the passage of blood-tinged mucus late in pregnancy. This “bloody show” results as the cervix begins thinning (effacement) with the concomitant extru-sion of mucus from the endocervical glands and a small amount of bleeding from small vessels in the area. Cervical effacement is common before the onset of true labor, when the internal os is slowly drawn into the lower uterine segment. The cervix is often significantly effaced before the onset of labor, particularly in the nulliparous patient. The mechanism of effacement and dilation and the vectors of the expulsive forces are demonstrated in Figure 8.1.