The Follicular Phase of the Menstrual Cycle

From the Start of Your Period to Ovulation

The female reproductive cycle is more complicated than just ovulating and having a monthly period. There's a lot going on all 28 days. The follicular phase of your menstrual cycle, also known as the proliferative phase, begins on the first day of your cycle, the day your period starts. This phase ends when a hormone called luteinizing hormone (LH) peaks and ovulation occurs, at which point you pass into the luteal phase of your cycle, at about day 14.

The follicular phase includes the menses phase.

What Happens During the Follicular Phase?

On the first day of your menses, the top layer of your thickened uterine lining (endometrium) begins to be shed. The endometrium was built up with fluids and nutrients in case a fertilized egg implanted. Without implantation and the hormones produced by the developing embryo and placenta, the lining is shed and you have menstrual bleeding. For most women, this last from three to five days.

After your menstrual flow has ended, the lining of the uterus is at its thinnest. Your levels of estrogen and progesterone are at their lowest. At around the fifth day of your cycle, increasing amounts of follicle-stimulating hormone (FSH) are released by the pituitary gland to signal the egg follicles to mature. These are located in the ovaries, with each follicle containing and nourishing an egg.

Around the eighth day of your cycle, the FSH levels drop, one of the developing follicles emerges as the dominant one, and all of the others begin to shrivel and disintegrate.

The dominant follicle begins to produce estrogen.

By the twelfth day, estrogen works to stimulate more LH production, which continues the maturation process of the follicle. Proliferation and thickening of the uterine lining begins, preparing your body for a possible pregnancy.

The follicular phase typically lasts about 14 days, although it is the most variable stage in length.

The luteal phase begins with a surge of luteinizing hormone and follicle-stimulating hormone, resulting in releasing the egg at ovulation.

The Follicular Phase and Your Fertility

Infertility can result if any of the hormonal triggers are not functioning in coordination during the follicular phase. Follicle growth and development are regularly tracked during fertility treatments. During superovulation, used during in-vitro fertilization (IVF), the goal is to stimulate the ovaries to develop several mature follicles at once.

In normal ovulation, between three and 30 follicles undergo accelerated growth during the follicular phase, but only one continues to develop to ovulation. When the egg is released, the other follicles are signaled to stop growing.

Controlled ovarian stimulation can be done to improve the chance of pregnancy, with more than one follicle developing and being released at ovulation. Clomiphene and human chorionic gonadotropin (hCG) are given to trigger ovulation. Pregnancy might be achieved through insemination. Semen can be transferred directly into the uterus so it bypasses the mucus plug that can block it. This brings an increased risk having a pregnancy with more than one fetus.

The eggs may be harvested to use for in-vitro fertilization during another round of superovulation if insemination doesn't work. Or, the attempt at insemination might be skipped altogether and the eggs can be harvested for in-vitro fertilization.

Sources:

Assisted Reproductive Technology (ART). National Institutes of Health. https://www.nichd.nih.gov/health/topics/infertility/conditioninfo/treatments/Pages/art.aspx.

Menstrual Cycle. Cleveland Clinic. https://my.clevelandclinic.org/health/articles/normal-menstruation.

Menstrual Cycle. Merck Manual. http://www.merckmanuals.com/home/women-s-health-issues/biology-of-the-female-reproductive-system/menstrual-cycle.

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