Written by Tamar L. Goulet, Ph.D.
You may think you’ve just got one monthly cycle controlling whether or not you conceive every month, but there are actually three—all doing their part to help you get pregnant.
If you’ve ever spent any time thinking about your menstrual cycle, chances are you think of it as one system that results each (approximately) month in either a period or a pregnancy. But if you focus more closely on what actually happens in a woman’s body, you’ll see there are really three different cycles at play, all working in tandem. In order for the winning trifecta of ovulation, fertilization, and implantation to happen—i.e., a successful pregnancy—the cycles that control the ovaries, uterus, and brain (hormones) all need to proceed in a predictable, intertwined fashion.
The Ovarian Cycle
Think of the ovaries as the part of your body that kicks things off as soon as they get the hormonal signal from the brain. Their first job: gearing up to release an egg. “It’s as if there’s a call for an audition for a Broadway show,” says Katharine O’Connell, M.D., an assistant clinical professor of obstetrics and gynecology at Columbia University in New York City. “Multiple small follicles, each of which contains an egg precursor, respond to the call and audition for the starring role,” which is to be the one dominant follicle that will release an egg that month. (There is an exception to this rule, though: Sometimes two eggs are released, which is how fraternal twins are conceived.) Depending on the woman, the “auditions” can last from seven to twenty-one days, as one follicle continues to grow and the rest fall behind. “Only one follicle gets the part and becomes dominant,” Dr. O’Connell confirms. “The ‘star’ then prepares for opening night,” or ovulation—the release of the egg from the follicle. Typically, ovulation occurs fourteen days before your next period.
After the egg is released, the tissue that encased the egg turns into the corpus luteum (or yellow body), which releases progesterone so the uterine lining can sustain an embryo. “If there’s no pregnancy, all this churning out of hormones stops, and the corpus luteum is reabsorbed,” Dr. O’Connell explains.
The Uterine Cycle
During the audition phase, if the follicles have been doing their work well, progesterone is helping the uterus to get ready to receive an embryo and give it a good home for the next nine months. “The uterine lining, or endometrium, starts out relatively thin, but thickens over roughly the second two weeks of a woman’s cycle,” Dr. O’Connell notes.
If fertilization occurs, the embryo needs to nestle into the endometrium; with the support of the corpus luteum, and later the placenta, it will thrive and grow. But, adds Dr. O’Connell, “If there’s no embryo, the lining starts to break down, which leads to your period.” Menstrual blood is the built-up uterine lining shedding because it’s not needed to nourish an embryo.
The Hormonal Cycle
All the action in the ovaries and uterus is triggered and coordinated by hormones sent from the pituitary gland in the brain and from the ovaries themselves. Beyond estrogen and progesterone, there are two more key hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Follicle stimulating hormone, as its name implies, stimulates follicles to start to grow. Luteinizing hormone, meanwhile, “is the one that spikes and triggers ovulation,” Dr. O’Connell says. “This LH surge is the spike we can test for in urine to know when ovulation is going to happen. We don’t test directly for ovulation, but for the hormone that causes it.”
In order for a woman to have regular periods and the best odds of conceiving, all three of the cycles need to do their work, maintaining a delicate balance of synchronization. If there’s a glitch in even one cycle, a successful pregnancy could be in jeopardy.
Luckily, fertility specialists are getting better at figuring out when hormone levels are off, how well your ovaries are functioning, and whether your uterine lining is the welcoming environment needed to nourish an embryo—and they’re offering more options to help overcome these glitches and improve your chances of getting all your cycles working flawlessly together.