Conception: The Incredible Journey

Our bodies are biologically programmed to help pregnancy occur in many different ways. For instance, a woman’s sexual desire is usually at its peak during her ovulation and just prior to her menstruation (Wilcox et al., 2004). During ovulation, a mucus plug in the cervix disappears, making it easier for sperm to enter the uterus, and the cer­vical mucus changes in consistency (becoming thinner and stretchy), making it easier for sperm to move through the cervix. The consistency of this mucus also creates wide gaps, which vibrate in rhythm with the tail motion of normal sperm, helping to quickly move the healthy sperm and detain abnormal sperm. The cervical mucus also helps filter out any bacteria in the semen. Finally, the female orgasm helps push semen into the uterus; once there, the orgasmic, muscular contractions of the vagina and uterus help pull sperm up toward the Fallopian tubes (pregnancy can certainly still occur, how­ever, without the woman having an orgasm). The consistency of the ejaculated semen also helps. Almost immediately after ejaculation, semen thickens to help it stay in the vagina. Twenty minutes later, when the sperm has had a chance to move up into the uterus, it becomes thin again.

With all the help our bodies are programmed to give, the process of getting pregnant may appear rather easy; however, this is not always the case. The process of becoming pregnant is complex, and things can and do go wrong. For example, the female’s immune system itself begins to attack the semen immediately after ejaculation, thinking it is un­wanted bacteria. But although many sperm are killed by the woman’s immune system,

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Figure 12.1 Number of births per 1,000 women between the ages of 15 and 44 years old by ethnicity. In the United States, minority women have more births than non-minority women. Source: Ventura et al., 2003.

this process is usually not a threat to conception. When a fertile woman engages in in­tercourse, 30% of the time she becomes pregnant, although a significant number of these pregnancies end in spontaneous abortion (Zinaman et al., 1996).

Because the ovum can live for up to 24 hours and the majority of sperm can live up to 72 hours in the female reproductive tract, pregnancy may occur if intercourse takes place either a few days before or after ovulation (A. J. Wilcox et al., 1995). Although most sperm die within 72 hours, a small number, less than 1%, can survive up to 7 days in the female reproductive tract (Ferreira-Poblete, 1997). Throughout their trip into the Fallopian tubes, the sperm haphazardly swim around, bumping into things and each other. When (and if) they reach the jellylike substance that surrounds the ovum, they begin wriggling violently. Although it is not clear how the sperm locate the ovum, pre­liminary research indicates that the ovum releases chemical signals that indicate its lo­cation (Palca, 1991).

Several sperm may reach the ovum, but only one will fertilize it. The sperm secretes a chemical that bores a hole through the outer layer of the ovum and allows the sperm to penetrate for fertilization. The outer layer of the ovum immediately undergoes a phys­ical change, making it impossible for any other sperm to enter. This entire process takes about 24 hours. Fertilization usually occurs in the ampulla (the funnel-shaped open end of the Fallopian tube; see Figure 12.2); after fertilization, the fertilized ovum is referred to as a zygote.

As we discussed in Chapter 3, the sperm carry the genetic material from the male. Each sperm contains 23 chromosomes, including the X or Y sex chromosome, which will

Uterus

determine whether the fetus is male or female. Other information is determined by both the male and female genes, including eye and hair color, skin color, and weight.

Approximately 12 hours after the genetic material from the sperm and ovum join to­gether, the first cell division begins. At this point, the collection of cells is referred to as a blastocyst. The blastocyst will divide in two every 12 to 15 hours, doubling in size. As this goes on, the cilia in the Fallopian tube gently push the blastocyst toward the uterus. Fallopian tube muscles also help to move the blastocyst by occasionally contracting.

Approximately 3 to 4 days after conception, the blastocyst enters the uterus. For 2 to 3 days it remains in the uterus and absorbs nutrients secreted by the endometrial glands. On about the 6th day after fertilization, the uterus secretes a chemical that dis­solves the hard covering around the blastocyst, allowing it to implant in the uterine wall (R. Jones, 1984). Implantation involves a series of complex interactions between the lining of the uterus and the developing embryo, and this usually occurs 5 to 8 days after fertilization. To facilitate implantation, the endometrium must have been exposed to the appropriate levels of estrogen and progesterone. Most of the time, implantation takes place in the upper portion of the uterus, and once this occurs the woman’s body and the developing embryo begin to exchange chemical information. Hormones are released into the woman’s bloodstream (these can be detected through pregnancy tests). If im­plantation does not occur, the blastocyst will degenerate and the potential pregnancy will be terminated.

It is fascinating that a woman’s body allows the blastocyst to implant when so many of her body’s defenses are designed to eliminate foreign substances. Apparently there is some weakening of the immune system that allows for an acceptance of the fertilized ovum (Nilsson, 1990). Some women do continually reject the fertilized ovum and expe­rience repeated miscarriages. We will discuss this in greater detail later in this chapter.

After implantation, the blastocyst divides into two layers of cells, the ectoderm and endoderm. A middle layer, the mesoderm, soon follows. These three layers will develop into all the bodily tissues. From the 2nd through the 8th weeks, the developing human is referred to as an embryo (EMM-bree-oh; although it may also be referred to as a “pre­embryo” prior to the embryonic stage). Soon a membrane called the amnion begins to grow over the developing embryo, and the amniotic cavity begins to fill with amniotic fluid. This fluid supports the fetus and protects it from shock, as well as assists in fetal lung development. The placenta, which is the portion that is attached to the uterine wall, supplies nutrients to the developing fetus, aids in respiratory and excretory func-

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