How Female Hormones Work in Fertility—And How IVF Doctors Use Them to Help Women Conceive

Fertility is a dance choreographed by hormones. For many women who embark on the journey of IVF, egg freezing, or embryo creation, understanding how their hormones function becomes essential—not just for peace of mind, but for making empowered, informed decisions.

Behind every injection, ultrasound, or blood draw is a strategy built on how female hormones work in a natural cycle—and how fertility doctors gently, and sometimes not so gently, manipulate these hormones to improve the chances of conception.  

Various female hormones at play during an IVF cycle

The Hormonal Symphony of a Natural Cycle

In a healthy menstrual cycle, four major hormones interact in a delicately timed rhythm: FSH (Follicle-Stimulating Hormone), LH (Luteinizing Hormone), Estradiol (E2), and Progesterone. Together, they regulate the maturation of eggs, trigger ovulation, and prepare the uterus to receive an embryo.

The cycle begins with FSH, which is produced by the pituitary gland. FSH stimulates the ovaries to grow a group of small fluid-filled sacs known as follicles, each of which holds an immature egg. In a typical cycle, one follicle becomes dominant and continues to grow, while the rest are reabsorbed. As follicles grow, they produce estradiol, a form of estrogen that thickens the uterine lining in preparation for possible implantation.

Once estradiol reaches a critical level, it sends a signal back to the brain to trigger an LH surge, which causes the dominant follicle to release its egg—ovulation. After ovulation, the follicle transforms into a structure called the corpus luteum, which produces progesterone, a hormone essential for maintaining the uterine lining and supporting early pregnancy.

When these hormones are functioning as they should, the body is primed for conception. But when time, age, or underlying conditions alter that rhythm, IVF offers an opportunity to step in and take the lead.

How IVF Doctors Use Hormones to Support Conception

In IVF, doctors aim to override the natural selection process that allows just one follicle to mature. By using medications, they encourage the ovaries to produce multiple mature eggs in one cycle, increasing the chances of retrieving good-quality eggs for fertilization.

Stimulation Phase: Growing Multiple Follicles

This process begins with injectable hormones like Gonal-F (which contains FSH only) or Menopur (a combination of FSH and LH). These medications bypass the brain’s signaling and directly stimulate the ovaries to grow several follicles at once.

As follicles grow, estradiol levels rise. Doctors monitor these levels through blood tests and ultrasounds, adjusting medication doses as needed. If the response is sluggish, doses are increased or LH is added. If too many follicles develop, the dose may be reduced to lower the risk of ovarian hyperstimulation syndrome (OHSS).

For women with high LH levels—common in conditions like PCOS (Polycystic Ovary Syndrome)—FSH-only medications are often used to avoid overstimulation. For those with low ovarian reserve or advanced reproductive age, adding LH through Menopur can help support the maturation process and improve egg quality.

Preventing Premature Ovulation

In a natural cycle, once estradiol reaches a certain level, the body releases LH, which triggers ovulation. In IVF, premature ovulation would mean losing all the eggs before retrieval, so doctors use medications to prevent this.

Two common medications are:

• Ganirelix: A GnRH antagonist that blocks the brain from triggering an early LH surge.

• Cetrotide: Another GnRH antagonist that works similarly, often used in the same protocols.

These medications allow doctors to control exactly when ovulation will occur, improving the timing of egg retrieval.

Timing Ovulation: The Trigger Shot

Once follicles are mature, doctors administer a trigger shot—typically hCG (human chorionic gonadotropin) or Lupron—to mimic the natural LH surge and prompt final egg maturation. This is a critical moment in the process, as timing the retrieval too early or too late can affect the quality of the eggs collected.

Luteal Phase: Preparing the Uterus

Because IVF bypasses natural ovulation, there’s no corpus luteum to produce progesterone. Instead, women are given progesterone supplements (via injections, pills, or vaginal gels) to support the uterine lining and encourage implantation after embryo transfer. In some cases, estradiol is also supplemented to stabilize the lining.

For Those Who Don’t Respond: A New Approach with Pills

Not every woman’s body responds well to injectable medications. Some have low ovarian reserve, high baseline FSH, or have undergone several rounds of IVF with poor follicular development. For these women, mild or minimal stimulation protocols—sometimes called "pill IVF"—offer an alternative.

In this approach, instead of daily injections, women take oral medications such as Clomid (Clomiphene Citrate) or Letrozole (Femara) to gently stimulate the ovaries. These drugs are often used in natural or mini-IVF cycles and can be particularly helpful for:

• Women over 38  

• Those with AMH <1.0 ng/mL  

• Poor responders to conventional IVF  

• Women who want a lower-cost, lower-risk approach  

1. Clomid

Clomid blocks estrogen receptors in the brain, making the brain think estrogen levels are low. This tricks the pituitary gland into releasing more FSH and LH, stimulating the ovaries to grow one or more follicles. It's been widely used for decades and is often the first line of treatment for ovulation induction. However, Clomid can sometimes thin the uterine lining due to its anti-estrogenic effects, which may reduce implantation chances in some women.

2. Letrozole

Letrozole belongs to a class of drugs called aromatase inhibitors, which block the conversion of androgens to estrogen. By lowering estrogen levels, the brain increases FSH production, stimulating the ovaries. Letrozole is often considered superior to Clomid for women with PCOS, because it doesn’t cause the same rise in LH and has a lower risk of thinning the endometrium.

In some IVF protocols, Letrozole is also used in combination with low-dose injectable FSH to strike a balance between effectiveness and safety.

Final Thoughts: Navigating the Science with Confidence

IVF is both an art and a science. The science lies in understanding the intricate interplay of hormones; the art lies in tailoring protocols to each individual’s response. Whether a woman needs high doses of injectable FSH, a combination of FSH and LH, or a gentler oral medication approach, the guiding principle remains the same: use hormones strategically to create the best possible chance for conception.

For women preparing for IVF, learning how these hormones function is a powerful first step. It's a way to move from fear and uncertainty toward clarity and control—because the more we understand our biology, the more confident we become in the choices we make for our fertility.