What Triggers Childbirth?

Many hormonal processes influence when labor begins.

To the dismay of women suffering through the aches and pains of the last month of pregnancy, many aspects of labor remain mysterious. Doctors don't understand exactly what causes labor to begin--or how to predict when labor will start. However, while science can't completely explain the complicated process of childbirth, research has identified a number of different factors that appear to play a role in triggering labor.

Infections

Viral and bacterial infections play a significant role in triggering labor in preterm--before 37 weeks--births, according to a 2007 survey of research on infectious causes of premature labor in "Seminars In Reproductive Medicine." The number one infectious cause of preterm births is bacterial infection in the uterus. The earlier a premature birth occurs, the higher the likelihood of bacterial infection in the amniotic fluid. Infections in the mother--such as pneumonia, malaria and periodontal disease--also increase the risk of premature delivery. The presence of viruses such as CMV, Epstein-Barr, herpes simplex and parvovirus B19 in the amniotic fluid increase the risk of negative pregnancy outcomes including premature labor and preterm rupture of membranes.

Placental Triggers

Levels of placental corticotropin-releasing hormone (CRH) increase significantly in a mother's blood before birth. A 2009 study in the "Journal of Clinical Endocrinology and Metabolism" hypothesized that rising levels of placental CRH trigger labor by increasing the ratio of estriol, a form of estrogen, to progesterone. Their findings that placental CRH levels were significantly higher at 26 weeks in mothers who had preterm deliveries support the role of this placental hormone in starting labor.

Maternal Triggers

Throughout most of a pregnancy, progesterone plays a critical role in preventing uterine activity. As the time for childbirth nears, maternal progesterone receptors become less sensitive to progesterone, which lowers the amount of circulating progesterone and allows the uterus to begin contracting. Maternal behaviors can also trigger labor. A 2005 study in "ACTA Obstetricia et Gynecologica" found that mothers who smoke face an increased risk of very early preterm births. The more they smoked, the greater their risk of giving birth prematurely.

Fetal Triggers

As a fetus's lungs mature, they produce "surfactant," a substance lining the lungs that helps people breathe. A 2004 study on mice in the "Proceedings of the National Academy of Sciences" found that as the lungs of fetal mice reached maturity--indicating a readiness to breathe air--they secreted an increasing amount of surfactant protein into the uterus, which helped stimulate the hormonal chain involved in labor. A fetus's adrenal gland may also help trigger labor by increasing the amount of cortisol it produces. Higher levels of cortisol help mature a baby's lungs and stimulate the production of substances, such as prostaglandins, involved in labor.

References

• "Seminars in Reproductive Medicine"; The Role of Infection and Inflammation in Preterm Birth; Roberto Romero, et al; June 2007
• "Journal of Clinical Endocrinology & Metabolism"; Patterns of Plasma Corticotrophin-Releasing Hormone, Progesterone, Estradiol and Estriol Change and the Onset of Human Labor; Roger Smith, et al.; March 2009
• "Nature News"; MicroRNAs Mediate an Early Birth; Joseph Milton; November 2010
• "Molecular Endocrinology"; Minireview: Fetal-Maternal Hormonal Signaling in Pregnancy and Labor; Carole R. Mendelson; May 2009
• "ACTA Obstetricia et Gynecologica"; Maternal Smoking and Causes of Very Preterm Birth; Nina B. Kyrklund-Blomberg, et al.; May 2005
• "Proceedings of the National Academy of Sciences of the United States of America"; Surfactant Protein Secreted By the Maturing Mouse Fetal Lung Acts as a Hormone that Signals the Initiation of Parturition; Jennifer C. Condon, et al.; April 2004

Resources

• Ohio State University Medical Center: Labor