Progesterone is a naturally occurring steroid hormone widely known as the “pregnancy hormone” due to its fundamental role in supporting gestation. When a person is trying to conceive or is newly pregnant, the level of this hormone often causes anxiety, especially if a blood test reveals a lower-than-expected reading. Many people who experience early pregnancy loss wonder if low progesterone was the reason for the miscarriage. Understanding the science behind this hormone, how it is measured, and its relationship to pregnancy viability can help address this concern.
The Essential Role of Progesterone in Early Pregnancy
Progesterone’s primary function in the first trimester is to create and maintain a hospitable environment within the uterus for the developing embryo. After ovulation, the temporary structure left behind on the ovary, called the corpus luteum, secretes this hormone in large quantities. Progesterone acts on the endometrium, the uterine lining, transforming it into a secretory state rich in nutrients and blood vessels to support the implanted embryo.
The hormone also works to keep the uterus relaxed by suppressing the muscular contractions of the myometrium, a function often referred to as maintaining uterine quiescence. This calming effect helps prevent the expulsion of the pregnancy in its early stages. Around the 8th to 12th week of gestation, the placenta gradually takes over progesterone production from the corpus luteum, a process known as the luteal-placental shift.
Defining and Testing for Low Progesterone Levels
The most common method for determining progesterone status in early pregnancy is a serum progesterone test, which measures the hormone concentration in the blood. Results are reported in nanograms per milliliter (ng/mL). The normal range is broad because levels vary significantly and fluctuate throughout the day. First-trimester levels are generally expected to be between 10 and 44 ng/mL, though some lab-specific reference ranges may extend higher.
A single low measurement is rarely conclusive due to the pulsatile nature of the hormone’s release. Healthcare providers often look for a pattern by ordering serial measurements over a few days, or by pairing the progesterone result with a human chorionic gonadotropin (hCG) level. Levels below 5 ng/mL are highly suggestive of an abnormal or non-viable pregnancy, such as a miscarriage or an ectopic pregnancy. Diagnosis is guided by the overall trend and clinical context, not a single number.
Correlation vs. Causation: Low Progesterone and Miscarriage Risk
The question of whether low progesterone causes a miscarriage or is merely a sign of a failing pregnancy is a central point of discussion. Clinical evidence suggests that in most instances of early pregnancy loss, low progesterone is a symptom of a non-viable pregnancy, rather than the primary cause. The majority of miscarriages are attributed to chromosomal or genetic abnormalities in the embryo, which prevent proper development.
When a pregnancy is not developing correctly, the embryo fails to signal the corpus luteum effectively to maintain hormone production. Consequently, the corpus luteum naturally regresses, and progesterone levels drop because the pregnancy is already destined to fail. In this scenario, the low hormone level is a biological marker indicating that the pregnancy is no longer progressing, not the event that triggered the loss. While progesterone is necessary to sustain the pregnancy, its absence usually reflects an underlying issue with the developing embryo.
Progesterone Supplementation: Medical Use and Efficacy
In cases where low progesterone is identified, supplementation is a common treatment, usually administered via vaginal suppositories, gels, or injections. This treatment is universally required for pregnancies conceived through In Vitro Fertilization (IVF). The hormonal stimulation used in IVF can impair the function of the corpus luteum, making supplementation necessary to provide the standard luteal phase support the body cannot produce naturally.
For spontaneously conceived pregnancies, the evidence is more nuanced, but research has identified specific groups that may benefit. Clinical trials have shown that progesterone supplementation does not significantly improve outcomes for the general population experiencing a threatened miscarriage. However, evidence suggests that progesterone can be helpful for those with a history of recurrent pregnancy loss (two or more miscarriages). Supplementation may also be offered to women with unexplained recurrent miscarriage who present with bleeding in the current pregnancy.