Biological Steps to Becoming Pregnant

There are many steps that must occur for a female to become pregnant. Understanding the process of how pregnancy occurs can be beneficial both for those who are trying to become pregnant, and those who wish to avoid pregnancy. For those that wish to avoid pregnancy, we highly recommend using both a barrier method of birth control, such as a condom, and a hormonal method, such as birth control pills. Using a condom properly will also help prevent the spread of sexually transmitted infections (STIs).

When trying to become pregnant, eating well, staying healthy, tracking ovulation, and age can play a role in the chances of pregnancy occurring. Within a year of trying to conceive, females younger than 25 years old have an 86% chance of becoming pregnant, females age 25 to 29 have a 78% chance of pregnancy, females age 30 to 34 have a 63% chance of pregnancy, and even females age 35 have more than a 50% chance of pregnancy.1 If a female older than age 35 has not conceived after trying for 6 months, it may be a good idea to see a fertility specialist, such as a reproductive endocrinologist, to help treat potential fertility problems.

In general terms, the journey to pregnancy begins when a male’s sperm fertilizes a female’s egg. This fertilization can happen in a variety of different ways, such as the following:

  • Unprotected vaginal intercourse, during which a male ejaculates into a female’s vagina.
  • Other sexual activity in which a male’s semen, whether contained in Cowper’s gland secretions (known as pre-cum) or in ejaculate, comes into contact with a female’s vulva.
  • Alternative forms of insemination, in which sperm, either from the female’s male partner or from a donor, are inserted into a female’s vagina or uterus using a syringe or other device. (This is an option for single females, female-female partnerships, or couples dealing with infertility).
  • Other fertility treatments, such as in vitro fertilization, where a female’s eggs are taken out of her body, fertilized with sperm, and then reinserted back into the uterus.2

This article will outline the steps it takes for a female to become pregnant, focusing specifically on the natural process, which occurs as a result of unprotected sex between a male and female partner. Some of the biological descriptions included may also apply to alternative fertility processes.

 

Step 1: Sperm and Egg Development

Females are born with 1 to 2 million immature eggs, about 400 of which are released over the course of her reproductive years, beginning with menarche and ending with menopause.3 She will not produce any more eggs during her lifetime. On the other hand, males are not born with pre-made sperm. Instead, they are constantly producing millions of sperm cells throughout their lifetime.3

Sperm Development

Sperm development begins in a male’s testicles, which are glands in the scrotal sac located beneath the penis. The testicles hang outside of the male’s body in order to regulate scrotal temperature, as healthy sperm is produced at 94 degrees Fahrenheit, which is about 4 degrees cooler than body temperature.3 It takes 64 to 72 days to create a new sperm cell, and due to the fact that about 100 to 300 million sperm cells are released with each ejaculation, a male is continuously producing sperm.4 Once the sperm cells are produced, they are stored in the epididymis. Just prior to ejaculation, the sperm cells are mixed with semen. The liquid portion of semen aids the sperm by providing it with nourishment, protection from the acidic vaginal canal, and direction by coagulating in the female’s vagina to form a barrier.4 This protection only lasts about 30 minutes before the semen returns to liquid form and any sperm that has not made it through the female’s cervix dribbles out of the female’s vagina.

Egg Development

The development of a female’s egg is much more complex than sperm production. As previously stated, a female is born with all of her eggs. These millions of immature eggs are contained in follicles (small, fluid-filled cysts), which are stored in the female’s ovaries (two almond-shaped structures on either side of the uterus). 3 Approximately once every month, the female’s hypothalamus sends a signal to the pituitary gland to release follicle-stimulating hormone (FSH), which prompts several follicles to develop into mature eggs.5 Only one of these several follicles will mature completely, with the others reabsorbing into the body.

Step 2: Ovulation

Once one egg has completely matured, it erupts from the follicle and pushes through the wall of the ovary. This process is called ovulation. The follicle of the mature egg develops into the corpus luteum, which releases a luteinizing hormone that helps thicken the lining of the female’s uterus.5, 6 The follicle then sends out signals for increased estrogen production, which indicate to the body that an egg is mature. Sometimes, though rare, more than one egg is released from a female’s ovaries within 24 hours. Fraternal twins can be the result of two released eggs becoming fertilized.5

Some females can feel a slight ache on one side of their lower abdomen during ovulation. Depending on which side the ache is coming from, these females can determine which of the two ovaries produced a mature egg.5 It is also possible for some females to track when they are ovulating by observing changes in their cervical secretions, which become stretchy, clear, and thin before and during ovulation.4, 5 Other possible identifying effects of ovulation are temperature changes, increased sex drive, light spotting, bloating, and heightened senses of taste or smell.5

Each ovary is attached to the uterus by a 4-inch-long, half-inch-wide canal called a fallopian tube. After the egg is released, it is picked up by the fimbriated (finger-like) end of the fallopian tube and stays in the tube for 24 hours waiting for a single sperm to fertilize it.3, 7 Tiny hairs called cilia help the egg travel through the fallopian tube towards the uterus.5 During the egg’s several day journey through the fallopian tube, the follicle that originally formed the egg releases signals causing the uterus to form an internal lining called the endometrium.5 This endometrium is full of blood and nutrients necessary to house and nurture an egg if it becomes fertilized.5

If there is no sperm to fertilize the egg, the egg will move through the uterus and disintegrate, the female’s estrogen and progesterone hormone levels will return to normal, the female’s body will shed the thick endometrium lining of the uterus, and the female’s period (menstruation) will begin.3, 6

 

Step 3: Fertilization

Many factors need to line up in order for fertilization, or the joining of a male’s sperm cell and a female’s egg, to occur. These factors include favorable environmental conditions, timing in the female’s menstrual cycle, fertility of the male’s semen, ability of the male’s sperm cells to penetrate the egg (also known as sperm capacitation), and development of the embryo. If all of these factors are present, fertilization between sperm and egg will most likely occur.

Environmental Conditions

The environment of a female’s vagina and cervix is under cyclical hormonal control, which must be favorable to admit sperm without destroying them.7 Every time a male ejaculates, the semen turns into a gel-like substance that provides the sperm protection from the female’s acidic vagina. After 20 to 30 minutes, this gel turns back into liquid, which allows some sperm to start their journey to the egg.7

Hormonal Timing 

The odds are stacked greatly against any one sperm. During most of a female’s menstrual cycle, the vaginal canal and cervical canal are inhospitable environments for a male’s sperm due to the thick consistency and acidity of the vaginal and cervical mucus. However, closer to ovulation, the female’s cervical mucus changes and becomes specially designed to transport sperm efficiently.

Because of the cyclical hormonal changes that occur throughout a female’s menstrual cycle, the biggest key to successful fertilization is to have the sperm meet the egg during the right time frame. The ideal time of fertilization is around ovulation when the egg is in the fallopian tube.

Male Fertility

After traveling through the vaginal canal, the strongest sperm make it to the cervix. From here, the sperm must travel through the uterus and into one of the fallopian tubes.7 Of the millions of sperm cells contained in each ejaculation, anywhere from just one to a couple hundred sperm cells arrive at the fallopian tube.5 If a male’s sperm count is significantly lower than about 500 million sperm cells per ejaculation, the male might be infertile so pregnancy may be nearly impossible to achieve naturally in these cases.

Since the human body functions to procreate, the egg that is released monthly from the ovaries waits for sperm cells in the fallopian tube for about a day. Additionally, sperm can survive in the female reproductive track for up to 5 days. Therefore, if the female’s egg and male’s sperm happen to meet at any point during that time frame, fertilization can occur.7 If the male’s sperm cannot survive long enough to meet the egg or if the male cannot seem to get the female pregnant after trying for several years, he may be experiencing infertility.

Sperm Capacitation

Even after a male’s sperm comes into contact with an egg, there are still many more steps that need to occur in order for a female to become pregnant. A couple dozen to a couple hundred of the strongest sperm surround the egg, trying to penetrate the egg’s hard outer cell membrane, called the zona pellucida.7 The rest of the sperm have either gotten trapped, lost (wrong fallopian tube), or have died along the way.3 As the remaining strong sperm surround the egg, the egg releases the hormone progesterone, which makes these sperm cells shed layers of proteins and experience a surge in activity.5 This process of sperm conversion is called capacitation.5, 7 Once the sperm make contact with the outside of the egg, the head of the sperm “pops,” which releases enzymes that allow the sperm to penetrate the hard outer shell of the egg.5

As soon as one sperm penetrates the exterior of the egg, the zona pellucida of the egg instantly undergoes a chemical reaction, preventing any other sperm cells from penetrating as well.4,7 Fertilization has now officially occurred, also referred to as conception.5 The egg absorbs the sperm, and the chromosomes of the egg and the sperm come together. This union of egg and sperm creates a zygote (fertilized egg) and this new zygote cell has a core of 46 chromosomes (23 from the sperm and 23 from the egg).

Embryonic Development

This new zygote cell begins to divide rapidly over the next few hours through a process called mitosis, and this expanded bundle of cells is now called an embryo.7 The sex of the embryo is already determined at this stage. If the fertilizing sperm has a Y chromosome, the embryo will develop into a male baby, while a fertilizing sperm with an X chromosome will develop into a female baby.6

The embryo continues to divide as it moves slowly through the fallopian tubes towards the uterus, a journey that takes about 3 to 4 days.6 During this time, the fertilized egg continues to develop and the uterus prepares to receive it.

 

Step 4: Implantation

By the time the fertilized egg reaches the uterus, it is a cluster of about 100 cells, and it forms a blastocyst.4 The blastocyst is a hollow structure, with the cells on the inside continuing to develop.5 At this point, it has been about 5 to 7 days since the sperm fertilized the egg.7 These developing cells in the blastocyst will eventually become the fetus, and the outer wall of the blastocyst will develop into the placenta and other tissues that will provide important nutrients for the growing fetus.5

After another day, the embryo then emerges from its membrane (the zona pellucida) and begins the process of implanting in the female’s uterus.7 When the blastocyst comes into contact with the endometrium lining of the uterus, hormones will be exchanged allowing the blastocyst to connect to the uterine wall. Simultaneously, the lining of the uterus gets thicker, and the cervix is closed off with a mucus plug that will stay in place until the end of the pregnancy term. Around the time of implantation, some females may notice spotting (light bleeding), which should only last about 48 hours.5, 6

In the coming weeks and months, the cells will continue to divide and exponential cell growth will occur.5 These cells will be very specialized and different, and within approximately 3 weeks the first nerve cells will develop.5, 6

Please note that irregularities may cause an embryo to implant somewhere other than the female’s uterus, such as in one of the fallopian tubes. This irregularity is called an ectopic pregnancy and it can be very dangerous. In these cases, the female will either need to take medication to stop the embryo from growing, or have surgery to prevent the fallopian tube from rupturing.3

 

Step 5: Detection

Naturally, about half of all fertilized eggs are lost before a female even knows that she is pregnant. This loss could be due to many reasons, including the blastocyst implanting but not growing, or the blastocyst growing but ceasing development.

After implantation occurs, a hormone called human chorionic gonadotropin (hCG) is present in the female’s blood.6 After about 3 to 4 weeks from the first day of the female’s last period, there are high enough levels of this hormone to be detected by pregnancy tests.6 It may be a few weeks until the female misses a period and even suspects that she may be pregnant.

 

Concluding Remarks

If a female misses her period or notices other pregnancy symptoms, a home pregnancy test can be purchased at a local drugstore. It is important to remember that no home pregnancy test guarantees 100% accuracy in its results. It is wise to wait a sufficient amount of time (about one week) after a missed period to take a home pregnancy test. It is also recommended to repeat the test in case of a false positive or false negative. If a female obtains positive results (meaning she is pregnant), then it is recommended that she visit a doctor’s office or clinic to obtain a laboratory test with more accurate results.

For some females, a positive pregnancy reading is exciting news, while other females might not be ready to have a baby or never wish to. Those faced with an unwanted pregnancy may have several options, such as aborting the pregnancy or carrying the fetus to term and considering adoption or raising the child. Although the fertilization steps included in this article focused on natural processes of conception, there are similar steps of implantation and detection associated with pregnancy occurring through insemination or other fertility treatment options. Overall, regardless of the journey to pregnancy, understanding the biological steps associated with conceiving a baby can be extremely beneficial to anyone engaging in sexual intercourse and wishing to make informed decisions related to reproduction.

 

References

1. "Weathering the Wait." What To Expect. N.p., n.d. Web. 4 Feb. 2017.

2. "How Pregnancy Happens - Getting Pregnant." Planned Parenthood. N.p., 15 Dec. 2016. Web. 5 Feb. 2017.

3. "Getting Pregnant: How Babies Are Made." BabyCenter. N.p., 14 Feb. 2017. Web. 5 Feb. 2017.

4. "How Fertilization Happens." What To Expect. N.p., n.d. Web. 5 Feb. 2017.

5. "Understanding the Conception Process." How Stuff Works. N.p., 17 Feb. 2011. Web. 5 Feb. 2017.

6. "Pregnancy and Conception." WebMD. WebMD, n.d. Web. 5 Feb. 2017.

7. "Conception: How It Works." UCSF Medical Center. N.p., n.d. Web. 5 Feb. 2017.

 

Last Updated: 15 February 2017.

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