Preterm labor, defined as birth occurring before 37 completed weeks of pregnancy, represents a significant global health challenge. Infants born prematurely face higher risks of serious health complications and mortality. Medical science uses hormonal intervention, specifically supplemental progesterone, as a strategy to reduce this risk in high-risk pregnancies. This article explores the scientific mechanisms and patient criteria supporting the use of supplemental progesterone to prevent premature birth.
Progesterone’s Role in Maintaining Pregnancy
Progesterone is often called the “hormone of pregnancy” due to its foundational role in maintaining gestation. Initially produced by the corpus luteum, the placenta later takes over production, ensuring a steady supply. Progesterone prepares the uterus by promoting the growth of the uterine lining (endometrium) necessary for implantation. It also helps the uterus remain in a relaxed state, a condition termed quiescence. This non-contractile environment is maintained until a “functional withdrawal” of progesterone action occurs near term, leading to the onset of labor and preventing premature uterine contractions.
Cellular and Muscular Mechanisms of Action
Supplemental progesterone acts primarily on the myometrium, the smooth muscle layer responsible for labor contractions. Progesterone binds to cell receptors, altering gene expression and reducing the excitability and sensitivity of myometrial cells. It suppresses contraction-associated proteins, such as gap junctions and oxytocin receptors. By dampening the electrical signaling between muscle cells, progesterone prevents the coordinated, rhythmic contractions that characterize labor.
Progesterone also acts as an anti-inflammatory agent, which is a mechanism for preventing preterm birth. Inflammation within the uterus and cervix is often a trigger for the onset of labor. The hormone suppresses the production of pro-inflammatory cytokines and signaling molecules, such as prostaglandins, which promote uterine activity. By blocking these inflammatory pathways, progesterone helps maintain the structural integrity of the cervix. This action keeps the cervix firm and closed, preventing the physical changes required for birth.
Identifying Candidates for Progesterone Therapy
Progesterone therapy is targeted toward individuals identified as having a high risk of preterm birth. The most established indication is a history of a previous spontaneous preterm birth, as delivering before 37 weeks significantly increases the risk of recurrence.
Another major criterion for eligibility is the discovery of a short cervix during routine ultrasound screening in the current pregnancy. A short cervical length, typically 25 millimeters or less between 16 and 24 weeks of gestation, is a strong indicator of increased preterm birth risk. This measurement is often obtained using transvaginal ultrasound.
The effectiveness of progesterone is most clearly demonstrated in these two high-risk groups. The treatment is not recommended for women carrying multiples, such as twins or triplets, as clinical trials have not consistently shown a benefit. Prescribing therapy relies on evaluating the patient’s obstetric history and current anatomical measurements.
Methods of Administering Progesterone
Progesterone for preterm birth prevention is administered via two main forms: vaginal preparations and intramuscular injections. The choice of method depends on the specific risk factor identified for the patient.
Vaginal administration, typically using suppositories or gels, is preferred for those with a short cervix. This route delivers the hormone directly to the cervix, maximizing local anti-inflammatory and muscle-calming effects.
For individuals whose risk is based on a prior history of spontaneous preterm birth, weekly intramuscular injections of 17-alpha hydroxyprogesterone caproate are used. This synthetic derivative provides a sustained, systemic dose of the hormone. The treatment regimen usually begins between 16 and 20 weeks of gestation and continues until about 36 weeks.