A normal menstrual cycle is important for conception. The monthly menstrual cycle refers to the maturation and release of a single egg (oocyte), and to the preparation of the uterus to receive the fertilized egg (embryo).
The hormones released during the menstrual cycle control the events that lead to conception. On the first day of the cycle, when menstruation (your period) begins, the uterus sheds its lining along with the unfertilized egg from the previous cycle. A typical cycle lasts for about 28 days (± 3-4 days) and is divided into three phases.
Follicular Phase – Days 1 to 13 (28 day cycle)
During the follicular phase, Follicle Stimulating Hormone (FSH) is released by the anterior pituitary gland, which acts on the follicles, causing their maturation and triggering their production of estrogen.
Estrogen acts on the endometrium causing it to thicken and preparing it to receive the fertilized egg.
The follicular phase is not generally stable in duration and ranges from 10 to 18 days. This phase determines the duration of the cycle as a whole. In 28 day cycles, it last 14 days. In shorter cycles (<28 days) the entire cycle is shorter by the number of days that the follicular phase is shorter. In longer cycles (>28 days) the cycle is longer by the number of days that this phase is longer.
Ovulatory Phase – About 14 days before the start of the next cycle
Just before ovulation, estrogen levels rise causing the secretion of Luteinizing Hormone (LH) by the hypothalamus, which acts on the dominant follicle and causes it to rupture and release the egg. During the reproductive years, a woman usually releases one mature egg each month and, in rare cases (< 1%) two eggs are released. This process is known as ovulation.
The cervical mucus is most receptive to sperm during this time and a woman has the best chance of conceiving just before and during ovulation.
Spermatozoa can survive for up to one week (with an average of three days) in the woman’s body, hence the period before ovulation is considered fertile. Fertile are also one to two days after ovulation because the egg has a lifespan of about 20 hours. If two oocytes mature, the second will be released within 24 hours from the first.
Ovulation does not necessarily occur in every cycle. Cycles without ovulation, referred to as anovulatory, occur naturally in young women at menarche, women who are nursing, those approaching menopause and those with cycles greater than 35 days.
It is a common misconception that the ovulatory phase begins around Day 14 of the cycle. In reality, it is 14 days before the start of the next cycle, which is not necessarily a 28-day cycle.
Day 1 of the cycle is marked by the first day of normal red blood flow. Once the duration of a woman’s cycle is determined, it is possible to pinpoint the ovulatory phase by subtracting 14 days from the beginning of the next cycle.
Luteal Phase – Days 15 to 28
In contrast with the follicular phase, the luteal phase is always 12-14 days in duration. During this time and with the influence of luteinizing hormone (LH) produced by the empty follicle, now called the corpus luteum, estrogen and progesterone is released preparing the endometrium for implantation of the fertilized egg.
The ovulatory phase of the menstrual cycle is the best time for fertilization. When a couple has intercourse during this time period, the sperm swims through the cervical mucus, up the uterus and along the fallopian tube where it meets the egg.
Despite the fact that a fertile man’s ejaculation contains millions of sperm, only one spermatozoa can fertilize a normal egg. The egg may be fertilized for about 24 hours after its release from the follicle.
If fertilization does not occur, the egg passes through the uterus and the corpus luteum ceases to function on around the 26th day of the cycle. The endometrium is then eliminated and the next cycle begins.
After fertilization, the embryo travels through the fallopian tube toward the uterus. Inside the uterus, it implants on approximately the 20th day of the cycle and continues to grow.
The corpus luteum produces progesterone to support the pregnancy.