The Process of Conception, from Ejaculation and Ovulation to Implantation

           

The process of “conception,”remarks Haeberle in his 1978 book The Sex Atlas, is “a very complex phenomenon that even now is far from being fully understood.” He goes on to note that a woman’s pregnancy is the result of a combination of several biological processes: fertilization, segmentation, and implantation. But before any of these processes take place, several other biological functions and processes must occur in a man and woman independently of one another, in preparation. And then, of course, there must be a sexual “dialogue” between a man and a woman.

In a woman, the biological function of the menstrual cycle is the first significant process to understand in the process of conception. This cycle is regulated by “intricate relationships among the hypothalamus and various endocrine glands, including the pituitary (located in the brain), the adrenal glands, and the ovaries and uterus.” ( p. 95. Crooks and Baur) The hypothalamus is responsible for monitoring the hormone levels in the bloodstream throughout the cycle, and releasing chemicals that in turn stimulate the pituitary gland to produce the two hormones that affect the ovaries: follicle-stimulating hormone (FSH) and leuteinizing hormone (LH).

This menstrual cycle is self-regulating and dynamic and is considered to have three phases: the menstrual phase, the proliferative phase, and the secretory phase. It is the proliferative phase of the menstrual cycle, ending in ovulation, which can be thought of as the first part of the process of conception in a woman.  During this phase, the pituitary gland increases production of FSH, which stimulates the developing ovarian follicles to mature and results in the production of several types of estrogen, which in turn causes the endometrial lining of the uterus to thicken, preparing itself for implantation. Once the level of ovarian estrogen in the bloodstream reaches a peak, the pituitary gland stimulates LH production, while suppressing the release of FSH. In response to the spurt of LH, the mature follicle ruptures and releases an ovum. It also stimulates the development of the corpus luteum (the part of the follicle that stays after the matured egg is released), which produces the hormone progesterone. The ovum then enters the open end of the adjacent fallopian tube almost immediately and begins traveling down toward the uterus, maturing while it is still in the upper third of the fallopian tube, to become ready to unite with a sperm cell, a process that takes only a few hours. This is when the sperm should arrive in order for fertilization to occur. The actual period that an egg and sperm can unite is less than twenty-four hours; however, the egg may unite with sperm that arrived up to five days before it did in the fallopian tube. During the time of ovulation, secretions of the cervical mucus are also increasing due to increased levels of estrogen, and the mucus becomes clear, slippery and stretchy, as well as more alkaline, which assists the sperm in their travels up through the uterus and into the fallopian tubes, as well as helps them survive.

In a man, the first crucial process that needs to occur, could be said to be an erection, which is coordinated by the autonomic nervous system. The nervous system sends out a message that causes the expansion of the arteries in the penis, increasing the rate of blood flow to the three erectile chambers of the penis, and causes said erection.  The next process in the male is that of ejaculation, “the process whereby the semen is expelled through the penis to the outside of the body.” (p. 128. Crooks and Baur) This process, like the erection, is a spinal reflex, and is essentially the result of a buildup of neural excitation to a critical level, which when reached, triggers the internal physical events, otherwise known as the two stages of ejaculation: the emission phase and the expulsion phase. 

During the first stage, or the emission phase, the prostate, seminal vesicles, and upper portions of the vas deferens, called the ampulla, undergo contractions, forcing their various secretions into the ejaculatory ducts and prostatic urethra. Meanwhile, the external and internal urethral sphincters close and trap the seminal fluid in the urethral bulb, which expands like a balloon. The experience of the emission phase in the male is known as “the point of no return” or “ejaculatory inevitability.”

            During the second stage, or the expulsion phase, the semen is forced out of the penis by the strong, rhythmic contractions of the muscles surrounding the root of the penis and the urethral bulb, as well as contractions along the entire urethral route. The internal urethral sphincter remains contracted, preventing escape of urine, while the external sphincter relaxes and allows the seminal fluid to pass through. The initial contractions around the base of the penis are rather strong and occur at close intervals. The majority of the seminal fluid is expelled in spurts that correspond with the contractions. A series of muscle responses typically follow, with a diminishing of intensity and a lengthening of time intervals between contractions. The entire expulsion stage usually takes place in three to ten seconds.

And, of course, a very important process in order for conception to occur naturally, is that of coitus, a process whereby a man inserts his erect penis into a woman’s vagina. Conception happens if a fertile man and a fertile woman engage in coitus, and the male ejaculates into the vagina, discharging his semen which carries between 200 and 500 million sperm cells, the timing is right, and one of those sperm cells unites with an ovum in a fallopian tube, where the egg’s fingerlike microvilli then draw the sperm to it.

This initial process is known as fertilization. Usually, several sperm reach the egg at the same time, but only one of them can penetrate it, because of a chemical change that occurs in the outer layer of the egg once a sperm cell has penetrated the ovum, which prevents any other sperm cells from breaking through. Once inside, the sperm cell loses its tail right away, and then swells and forms a nucleus, which then unites with the egg’s nucleus. The twenty-three chromosomes from each nucleus join together, creating a new cell that contains forty-six chromosomes, combining the inheritable characteristics of the man and the woman. This cell is called the zygote.

The process that follows is that of segmentation, which refers to the growth of the zygote by cell division, a process that occurs within a few hours after the nuclei of the sperm and egg have united. The zygote cell divides into two cells, then four, eight, sixteen, etc. The zygote is transforms into a cluster of cells called a morula, which actually resembles a mulberry. The morula moves slowly down toward the uterus, arriving in about three days, by which time, it has developed into a hollow ball of cells called the blastocyst. 

The final biological process involved in conception is the attachment of the blastocyst to the lining of the uterus. Once the blastocyst arrives in the uterus, it continues to develop for another three to four days, before it is ready to attach itself. The attachment to the uterine lining begins about one week after fertilization has occurred, and after about one more week, the blastocyst is completely buried in the endometrium. This entire process is called implantation, and it accomplishes pregnancy.  If the zygote has not transformed into a blastocyst by the time it reaches the uterus, or if the uterine lining is not prepared when the blastocyst arrives in the uterus, there will be no implantation. The cluster of cells will die and disintegrate.

Occasionally, the zygote splits up into two separate cells, that continue to grow independently and find their way separately to the uterus, or the blastocyst divides and grows during implantation into two separate embryos, resulting in a twin pregnancy. Because, they both originated from the same sperm and egg, they will be identical: of the same sex, and looking alike in every other aspect. Also, in the event of multiple ovulation, the release of several eggs at the same time, more than one egg may be fertilized and implant into the endometrium, again resulting in multiple pregnancies. If there are two, they are referred to as fraternal twins, three are triplets, four are quadruplets, and so on. They may be of different sex and they will not resemble each other any more than other brothers and sisters, because they come from a different egg, fertilized by a different sperm cell.

 

References

 

Crooks, Robert. Baur, Karla. Our Sexuality. PacificGrove:Wadsworth Group. 2002. pp 687

 

 

Haeberle, Erwin. The Sex Atlas. New York: The Seabury Press. 1978.  pp 509.