Progesterone is a naturally occurring steroid hormone central to the female reproductive system and the establishment of pregnancy. It is primarily produced by the corpus luteum, a temporary structure that forms in the ovary after ovulation. Progesterone’s purpose is to prepare the body for conception and sustain a pregnancy. Due to its actions, progesterone is frequently prescribed as a supplement for individuals undergoing fertility treatment or those at risk of certain pregnancy complications. The question of whether this supplementation increases the chance of conceiving twins or other multiples is a common concern.
Progesterone’s Function in Reproductive Health
Progesterone manages the uterine lining, or endometrium, during the menstrual cycle. After ovulation, progesterone levels rise significantly, transforming the endometrium from a proliferative state to a secretory state. This causes the lining to thicken and become enriched with blood vessels and glandular secretions, creating a receptive environment for a fertilized egg to implant.
If pregnancy occurs, the hormone continues to be produced, first by the corpus luteum and later by the placenta, to maintain the thick uterine lining. This sustained production prevents menstruation and relaxes the muscle tissue of the uterus, preventing contractions that could lead to an early miscarriage. In fertility treatments, supplementation supports this post-ovulation phase, known as the luteal phase, especially when natural production is compromised by medication or procedures like in vitro fertilization (IVF).
Progesterone supplementation is prescribed for fertility and pregnancy support. It ensures adequate preparation of the uterine lining before an embryo transfer in IVF cycles. It is also used to reduce the risk of early miscarriage by strengthening the uterine environment. Progesterone’s actions focus entirely on preparing and supporting the uterus; it does not stimulate the ovaries to release more than one egg.
Progesterone Supplementation and Multiple Birth Rates
The direct answer is no; progesterone supplementation does not cause multiple gestations. Scientific consensus confirms that the hormone, when used for luteal phase support or early pregnancy maintenance, does not increase the likelihood of conceiving fraternal or identical twins. Progesterone is a maintenance hormone, not a stimulation hormone; its function is to support a pregnancy that has already begun, not to initiate the release of multiple eggs.
Fraternal twins result from the fertilization of two separate eggs released in a single cycle, a process called hyper-ovulation. Progesterone does not induce hyper-ovulation or multi-follicular development. Identical twins result from a single fertilized egg splitting into two, which is a spontaneous and random event unrelated to hormone levels, including supplemental progesterone. Progesterone is prescribed to support the uterine environment, regardless of whether it contains one or multiple embryos.
Distinguishing Causation From Association in Fertility Treatments
The confusion about progesterone and twin rates stems from its association with fertility treatments that do increase the risk of multiple births. The high rate of twins and other multiples is caused by the underlying procedures used to achieve pregnancy, not the progesterone prescribed afterward. Progesterone supplementation is the final step in a treatment protocol that has already created the potential for a multiple gestation.
Treatments that actively stimulate the ovaries, such as Ovulation Induction (OI) using drugs like clomiphene citrate or gonadotropins, are the actual cause of the increased twin rate. These medications encourage the ovaries to develop and release multiple mature eggs in a single cycle, which can lead to fraternal twins if more than one egg is fertilized. For example, gonadotropins can lead to a twin pregnancy rate as high as 30%.
In assisted reproductive technologies, like In Vitro Fertilization (IVF), the risk of multiples is tied to the treatment protocol, specifically the number of embryos transferred. If the medical team transfers two embryos to maximize the chance of pregnancy, and both implant, a twin pregnancy results. Progesterone is then prescribed to support the uterine lining for these transferred embryos, but the hormone itself did not create the embryos or determine their number.
In these scenarios, progesterone compensates for the disruption of natural hormone production caused by preceding ovarian stimulation or egg retrieval procedures. Therefore, the hormone is an association—a supportive treatment following a procedure—rather than the cause of the multiple birth. Multiple births are a consequence of the fertility drugs that stimulate egg release or the clinical decision to transfer multiple embryos.