Progesterone is often called the “pregnancy hormone” due to its fundamental role in establishing and maintaining a healthy gestation. This steroid hormone prepares the uterine lining, the endometrium, making it receptive for the fertilized egg to implant. Beyond implantation, progesterone acts as a powerful muscle relaxant, suppressing spontaneous uterine contractions. Understanding when the levels of this protective hormone naturally change is central to grasping the biology of a typical pregnancy.
Progesterone’s Role in Early Pregnancy Maintenance
The initial source of progesterone is the corpus luteum, a temporary structure that forms in the ovary after ovulation. If conception occurs, human chorionic gonadotropin (hCG) signals the corpus luteum to continue producing large amounts of progesterone instead of degenerating. This production supports the pregnancy through its very early stages.
A significant event in the first trimester is the luteal-placental shift, marking the handover of progesterone production from the ovary to the developing placenta. This transition typically begins around seven weeks of gestation and is completed by about 10 to 12 weeks. The placenta rapidly increases its capacity to synthesize the hormone, eventually becoming the dominant source for the remainder of the pregnancy.
If the corpus luteum fails before the placenta is fully functional, a temporary drop in progesterone levels can occur. A decline during this narrow window of time poses a risk, as insufficient hormonal support may compromise the uterine lining. In a healthy pregnancy, the placental takeover ensures levels quickly resume their upward trajectory.
The Physiological Drop Signaling Labor
The most anticipated change in progesterone action occurs in the final stages of pregnancy, signaling the onset of labor. Throughout the second and third trimesters, progesterone levels remain extremely high, maintaining the uterus in a quiescent, relaxed state until full-term development.
In humans, a sharp, systemic drop in circulating progesterone levels, as seen in many other mammals, is generally not observed immediately before birth. Instead, the process is best described as a “functional progesterone withdrawal.” This involves a shift in the hormone’s effectiveness rather than a massive fall in its concentration.
As the pregnancy nears its end, the balance between uterine-calming progesterone and contraction-promoting hormones changes. The uterine muscle tissue becomes less responsive to high progesterone levels due to changes in receptor sensitivity. Simultaneously, the uterus increases its sensitivity to hormones like oxytocin and prostaglandins, which initiate contractions. This functional shift permits the uterus to transition to active labor, typically around 37 to 40 weeks gestation.
Clinical Implications of Abnormal Drops
A progesterone drop that occurs prematurely or a level that fails to rise adequately can have significant clinical consequences. In the first trimester, a rapid decline often reflects an underlying issue, such as an unviable pregnancy or abnormal implantation. Low serum progesterone levels in early pregnancy are frequently associated with threatened or inevitable miscarriage.
For women with a history of recurrent early pregnancy loss or those undergoing fertility treatments, insufficient progesterone may be a concern. In these scenarios, medical intervention, such as progesterone supplementation (vaginal suppositories or injections), is sometimes used to support the uterine lining. The goal is to artificially maintain the hormone’s protective effects to help sustain early gestation.
A drop in progesterone effectiveness later in pregnancy, prior to 37 weeks, is also linked to an increased risk of preterm labor. For women who have previously delivered prematurely or have a shortened cervix, progesterone supplementation can be administered weekly during the second and third trimesters. This preventative measure helps maintain uterine quiescence and reduces the likelihood of early contractions.