The ovaries, two small organs often associated only with monthly cycles, undergo profound transformations immediately upon conception, playing an active role in the initial weeks of pregnancy. While the uterus commands most of the attention during gestation, the ovaries are responsible for orchestrating the hormonal environment necessary for the pregnancy to take hold and survive its earliest stages. Before pregnancy, the ovaries release an egg each month and produce hormones that prepare the body for potential conception. With the arrival of a fertilized egg, this function shifts dramatically, focusing entirely on sustaining the new life.
The Ovaries’ Early Mandate (Corpus Luteum Function)
The primary ovarian function during the first trimester is centered on a temporary structure called the corpus luteum, or “yellow body,” which forms from the remnants of the follicle that released the egg. If fertilization occurs, the developing embryo begins to secrete human chorionic gonadotropin (hCG), a hormone that “rescues” the corpus luteum from its usual programmed decay. This structure then becomes the sole source of high-level progesterone and a moderate amount of estrogen, a function that is required for the pregnancy to continue.
The progesterone produced by the corpus luteum maintains the uterine environment. It induces the decidualization of the endometrium, transforming the uterine lining into a thick, nutrient-rich layer to support the conceptus. Progesterone also prevents the uterine muscle from contracting, which helps protect the developing embryo from being expelled. Without this sustained hormone production, the uterine lining would shed, resulting in menstruation and loss of the pregnancy. The corpus luteum acts as the body’s life support system for the embryo until another organ system is mature enough to take over the responsibility.
Physical Changes and Functional Inhibition
As the corpus luteum ramps up its hormone production, the ovaries also undergo physical changes and a complete inhibition of their reproductive cycling. The high levels of estrogen and progesterone circulating in the bloodstream create a powerful feedback loop that suppresses the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary gland. This hormonal suppression immediately prevents any further follicles from maturing and stops ovulation for the duration of the pregnancy.
Physically, both ovaries may appear slightly enlarged due to increased blood flow, a condition known as increased vascularity and edema. The ovary that hosts the corpus luteum is typically the most noticeably enlarged. The corpus luteum itself often presents as a functional ovarian cyst, which is a common and normal finding on early pregnancy ultrasounds.
These cysts are usually benign and represent the active, healthy structure that is producing the hormones needed to sustain the pregnancy. The size of the corpus luteum can range widely, sometimes reaching up to five centimeters in diameter, but it is rarely a cause for concern. The presence of an ovarian mass in early pregnancy is usually a temporary structure rather than a pathology.
The Hormonal Handover to the Placenta
The corpus luteum’s active phase is limited to the first several weeks of gestation, after which endocrine responsibility shifts to the developing placenta in a process known as the luteal-placental shift. This transition occurs when the placenta, specifically the syncytiotrophoblast cells, matures sufficiently to begin synthesizing its own large quantities of progesterone and estrogen. The shift typically begins around seven weeks of gestation and is usually complete between the ninth and twelfth week.
Once the placenta assumes this role, it becomes the definitive and major source of progesterone for the remainder of the pregnancy. The timing of this handover is a significant milestone, as it means the pregnancy is no longer solely dependent on the ovary’s temporary structure. Following this shift, the corpus luteum begins to regress, or involute, though it may remain visible on imaging.
After the shift, the ovaries become largely quiescent, maintaining their size but ceasing their role as the primary endocrine driver of the pregnancy. This hormonal transition is usually seamless in a natural pregnancy, but it is a closely monitored period in assisted reproductive technology cycles where the corpus luteum may not be present or fully functional.