Ovaries
Correct function of the ovaries (through hormones) regulates the menstrual cycle and allows for ovulation and successful implantation. This also occurs through the development and survival of the corpus luteum (CL).
The CL is a critical, temporary organ in normally the left or right ovary that secretes progesterone after ovulation, allowing the blastocyst to implant. The placenta takes over the CL's hormonal responsibility between 7 and 12 weeks of pregnancy. The CL then regresses as it is no longer needed for pregnancy.
Progesterone supplementation is often recommended for women with CL dysfunction, but research remains inconsistent regarding supplementation's effect on early pregnancy as well as optimal timing and dosage.
Background
Women have two ovaries; one on either side of the Fallopian tubes, which are connected to the uterus. Each ovary is estimated to be about the size of a golf ball, or about 2.0 cm in width, 3.5 cm in length and 1.0 cm in thickness.
The ovary has two functions – to secrete hormones and to house egg cells, or oocytes, that are released during ovulation.
Pregnant women will likely receive a transvaginal ultrasound during their first trimester, where assessment of the ovaries is included to look for any potential abnormalities and ensure the pregnancy is continuing as expected.
The main change conducted within the ovaries during pregnancy is the development of the corpus luteum (CL) and the production of progesterone, which is critical to the embryo’s survival.
Corpus Luteum
The CL develops from the follicle that released the ovulated egg, and produces hormones for establishing and maintaining pregnancy after ovulation. Although the main hormone is progesterone, it also produces estrogen and relaxin.
If pregnancy does not occur, the CL regresses to allow for menstruation and the reproductive cycle starts over. If a woman becomes pregnant, the CL will continue to secrete progesterone to provide the correct uterine environment. The CL “knows” a pregnancy occurred through the release of HCG into the bloodstream after implantation.
Progesterone from the CL is crucial to the survival of the pregnancy. If the CL is removed through surgery or regresses too soon, uterine contractions rapidly begin, and failure of implantation or miscarriage may result.
The production of progesterone from the CL is eventually taken over by the placenta, with the transfer process beginning and ending anywhere between 7 to 12 weeks of pregnancy.
Because the placenta takes over full control by 12 weeks, the ovaries of a pregnant woman can be removed after the first trimester (if necessary) without causing pregnancy failure.
Luteal phase insufficiency (LPI) is a condition in which a lack of progesterone is produced by the CL after ovulation, which prohibits implantation. Women with this condition may be given progesterone supplementation, generally until the 10-week mark (debated), for the reasons described above.
LPI itself is controversial; although progesterone supplementation is effective at reducing miscarriage in women with CL dysfunction, some researchers believe supplementation may only be needed until successful implantation. Therefore, the optimal route, dose, and time frame for progesterone supplementation for LPI requires further research.
Relaxin is produced by the corpus luteum around 6 weeks of pregnancy to help loosen and expand the pelvic bones, muscles, joints, and ligaments. The placenta produces lower amounts of relaxin after the corpus luteum degenerates, so blood levels of the hormone fall after the first trimester. Relaxin is only made during pregnancy.
If a pregnant woman produces little or no relaxin (such as those with no corpus luteum), it does not appear to affect birth outcomes. It is possible that women without relaxin still get the same loosening affect from progesterone and estrogen, just to a lesser degree.
Cysts
The majority of ovarian cysts detected in early pregnancy are benign and resolve on their own (corpus luteum cysts are very rare). Very few continue as pregnancy progresses or require treatment, although it is possible that some may require intervention.
The ovaries are examined during routine ultrasound examinations during pregnancy, and HCPs will inform women if any cysts were identified.
Action
Women should talk to their HCP about any concerns they have regarding CL function or their progesterone levels during early pregnancy.
Because there are conditions associated with the ovaries that can arise during pregnancy (torsion, cyts, masses) women should always call their HCP anytime they experience abdominal pain, so certain conditions can be ruled out.
Gaps
Is it possible for one CL to fail but another (subsequent pregnancy) does not? Or is CL failure consistent in women who experience this event?
Resources
Ovaries: Overview (Society for Endocrinology)