The corpus luteum is a temporary structure that forms on an ovary after an egg is released during ovulation. This mass of cells plays a role in establishing and maintaining early pregnancy.
The Function of the Corpus Luteum in Early Pregnancy
The primary role of the corpus luteum is to produce progesterone, a hormone that prepares the uterine lining for implantation. This hormone thickens the endometrium, making it a suitable environment for a fertilized egg. Progesterone also supports the growth of the early embryo and helps prevent uterine contractions that could disrupt the pregnancy.
This structure acts as a temporary support system for the pregnancy by ensuring the uterine wall remains thick and rich in blood vessels. It continues to produce progesterone until the developing fetus can produce sufficient levels to sustain the pregnancy. Without adequate progesterone, the uterine lining might not develop properly, potentially making successful implantation less likely.
Typical Corpus Luteum Size and Measurement
In early pregnancy, the corpus luteum is often identified as a cystic mass. Its diameter can range from just under 2 centimeters to 5 centimeters. During 5 to 9 weeks of gestation, the average diameter has been observed to be around 1.9 to 1.93 cm, showing little variation.
The most accurate way to measure and visualize the corpus luteum is through a transvaginal ultrasound. On an ultrasound scan, it often appears as a cyst with thick walls, a crenelated inner margin, and internal echoes, sometimes showing a “ring of fire” appearance with color Doppler imaging due to its peripheral vascularity. Healthcare providers typically focus more on its presence and function rather than its exact size within this expected range.
Interpreting Atypical Corpus Luteum Size
When a corpus luteum is observed to be smaller than average, it might be associated with lower progesterone production. A smaller corpus luteum could suggest it is not producing enough progesterone to adequately support the pregnancy. Healthcare providers often correlate ultrasound findings with blood tests to measure progesterone levels, providing a more complete picture of its function.
Conversely, a corpus luteum that grows larger than average is often referred to as a corpus luteum cyst. These cysts are common in pregnancy and are generally harmless, often indicating robust hormone production. They typically do not cause symptoms, but some individuals might experience mild, one-sided pelvic discomfort. Most resolve on their own without intervention, but rarely, a large cyst might rupture, causing internal bleeding, or lead to ovarian torsion, where the ovary twists, requiring medical attention.
The Lifecycle of the Corpus Luteum
The corpus luteum’s function in pregnancy is temporary, peaking around 7 to 9 weeks of gestation. As the pregnancy progresses, the placenta begins to develop to take over the production of progesterone and other hormones. This transition occurs between weeks 8 and 12 of pregnancy.
After the placenta assumes its role, the corpus luteum starts to regress. It becomes non-functional by the end of the first trimester, around 12 weeks of gestation. If fertilization does not occur, the corpus luteum breaks down approximately 10 to 14 days after ovulation, leading to a decrease in progesterone and the start of menstruation.