The follicular phase

The ovarian cycle has a total duration of about 28 days and is divided into a follicular phase which begins on the first day of the menstrual cycle and ends when ovulation begins, into a subsequent luteal phase.

We remember that:

  • the follicle  is a microscopic sac-shaped cavity,
  • the ovarian cycle,  has a total duration of about 28 days and is divided into an initial follicular phase and a subsequent luteal phase,
  • ovulation  is the moment during which the egg is expelled from the ovaries.

The follicular phase (also called pre-ovulatory or proliferative phase) is defined as that phase of the ovarian cycle which begins on the first day of the menstrual cycle and ends with ovulation.

During the follicular phase, the pituitary gland (a gland found in the skull) secretes a hormone that stimulates egg cells (ova) to grow in the ovaries.

One of these egg cells begins to mature in a “sack” structure called  a follicle: it takes 13 days for the egg cells to reach maturity.

After ovulation, the ovarian incision rapidly closes, while the rest of the follicle retracts and forms a corpus luteum. . Source: Biological Activations.

After ovulation, the ovarian incision rapidly closes, while the rest of the follicle retracts and forms a corpus luteum. Source: Biological Activations

As the egg cell (ovum, or secondary oocyte) matures, its follicle secretes a hormone that stimulates the uterus to develop a lining of blood vessels and soft tissue called the  endometrium .

The phases of the menstrual cycle

The menstrual cycle day count starts on the first day of your period. The most common cycle length for women is, as we said, 28 days.

The entire duration of the menstrual cycle can also be divided into the following four main phases:

  • the menstrual phase, from the 1st to the 5th day;
  • the follicular phase, from the 1st to the 13th day;
  • the ovulatory phase, 14th day;
  • the luteal phase, from the 15th to the 28th day.

Follicular phase, and fertility

During the follicular phase, a woman’s basal temperature drops to the lows of the pre-ovulatory temperature range.

After ovulation, the temperature will instead increase up to the maximum degree, typical of the post-ovulatory period, and the woman will enter the  luteal phase , which lasts until the end of the cycle.

During the second half of the follicular phase, estrogen levels rise so that your body can develop the endometrium (or lining of the uterus) into which the egg will implant if fertilization occurs.

While luteal phase length is generally constant from cycle to cycle (may vary at most by a day or two), follicular phase length is less predictable.

This is because many women don’t always ovulate on the same day each cycle.

So, if you want to predict your next period most accurately, you should do so based not on the overall lengths of previous cycles, but on the lengths of previous luteal phases.

So for example,

  • if your luteal phase is usually around 15 days,
  • and if in your current cycle you have confirmation of ovulation thanks to a lasting change in basal temperature on day 13,
  • you can expect your period to start around day 29 of your cycle (counting that day as day 1 of your next cycle, of course).

Its three sub-phases

From a physiological point of view, the follicular phase is divided into three different sub-phases, each characterized by different criticalities and specificities:

the prenatal stage

  • It lasts between 3 and 5 days.
  • In this phase, the primordial follicle becomes a primary follicle and the so-called zona pellucida forms around the oocyte.
  • At this stage, some cells of the ovarian stroma differentiate to form the outer layer of theca cells ( endocrine cells  of the ovary made up of connective tissue).
  • At the end of the prenatal phase, luteinizing hormone (LH) receptors appear on the thecal cells, and follicle-stimulating hormone (FSH) receptors on granulosa cells: both of these receptors are essential for the continuation of oogenesis. 

the antral sub-phase

  • It lasts between 8 and 12 days.
  • In this phase the maturation of the follicle takes place in a hormone-dependent way, going through 7 successive maturational stages, distinguished mainly on the basis of the different follicular size. 

the pre-ovulatory sub-phase

  • It lasts about 37 hours, and precedes the ovulation event.
  • In this phase the resumption of meiosis takes place (meiosis is the process by which the female egg cells and the male spermatozoa mature) with the detachment of the secondary oocyte (also called oocyte, or oocyte) from the wall, which is free to floating in the antral fluid.

In this stage, the pre-ovulatory follicle increases its volume considerably.

Katherine Johnson, M.D., is a board-certified obstetrician-gynecologist with clinical expertise in general obstetrics and gynecology, family planning, women’s health, and gynecology.

She is affiliated with the Obstetrics and Gynecology division at an undisclosed healthcare institution and the online platform,

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