The Yellow Body: Important for Menstruation...and Pregnancy
You’ve probably never heard of the corpus luteum (“yellow body” in Latin), but this tissue that’s produced—and discarded—by the ovary each month is the unsung hero of the menstrual cycle and the first trimester of pregnancy.
Corpus luteum sounds like something sinister and deadly. But this ovarian tissue actually performs a crucial role in maintaining a normal menstrual cycle and then protecting a pregnancy if necessary.
For the first half of the ovarian cycle—the follicular phase—the body prepares for ovulation. (There are actually two reproductive cycles. The “ovarian” cycle relates to things that happen in the ovary. The “menstrual” cycle relates to things that happen in the uterus.) Then, during ovulation, the egg is released from the ovary, leaving behind the tissue that surrounded it. This discarded tissue is the “yellow body,” or, more scientifically, the corpus luteum. In humans, the tissue has an orange tint, but it was named after studies in cows, whose corpus luteum is yellow. The second half of the ovarian cycle is called the luteal phase, named after this yellow body.
“The cycle is set up for pregnancy,” explains Michael A. Thomas, MD, director of the division of reproductive endocrinology and infertility at the University of Cincinnati Medical Center. Part of this preparation is building up the embryo’s “nest,” the uterine lining, which goes from “thin carpeting to plush carpeting,” as Dr. Thomas explains. The corpus luteum has a key role in maintaining this plush carpeting by secreting the hormone progesterone.
If an egg is not fertilized, the corpus luteum lingers for 13 or 14 days. It maintains progesterone levels that keep the uterine lining thick. At about 14 days, the yellow body disintegrates, and progesterone levels steadily drop to the point where the uterine lining cannot be sustained. The lining is shed, and the woman has her period. The corpus luteum is the reason a woman menstruates 13 or 14 days after ovulation if she’s not pregnant.
But if the egg is fertilized, the yellow body will linger much longer; it plays a critical role in early pregnancy. It takes an egg about seven days to travel from the ovary, through the fallopian tube, to the uterus. The corpus luteum provides the signal so that by the time a fertilized egg reaches the uterus, the lining is ready to accept it, and the egg implants.
The yellow body’s role doesn’t end there. “If a woman gets pregnant,” explains Dr. Thomas, “progesterone [secreted by the corpus luteum] maintains the uterine lining for eight to ten weeks, after which the placenta makes progesterone.”
Problems with the corpus luteum can cause menstrual cycle irregularities (called luteal phase defects), and difficulties getting or staying pregnant. “If a woman’s menstrual cycle is short,” Dr. Thomas explains, “it may mean the corpus luteum is not lasting an appropriate amount of time.” And since the yellow body controls progesterone production and maintains that production for the first eight to ten weeks of pregnancy, the embryo needs a properly functioning yellow body to survive.
While it may not be as well known a part of the human reproductive anatomy as the ovaries or the uterus, or even the placenta, the corpus luteum—the yellow body—plays a key role in a normal menstrual cycle and a healthy pregnancy.
A version of this article originally appeared in the Winter 2006 issue of Conceive Magazine.