Fertilization is a process through which humans reproduce their offspring. The male produces the sperm while the woman makes the egg, or ovum. When sperm are ejaculated, they use their flagella to move towards the egg, which they then surround in an attempt to reach the oocyte. The sperm head contains enzymes that help them penetrate the membrane of the egg. However, the egg normally allows only one sperm to be fertilized. Other unnecessary sperm die within 48 hours. After the sperm enters the egg, its head fuses with the cytoplasm of the egg. Fusion of the chromatin of the sperm and the egg is called the pronuclei, each of which contains a haploid genome. These genomes eventually develop into chromosomes. After the pronuclei fuse, the cell division starts. The cell division results in a diploid organism. The zygote then becomes a blastocyst when it contains around a hundred cells. This blastocyst floats for days in the uterine lining. Pregnancy officially begins when the blastocyst plants itself in the wall of the uterine. The blastocyst continues to develop and transforms into a human baby. 
Twins are often born. There can be fraternal, identical, conjoined, or semi identical twins. Use of reproductive drugs raise the possibility of bearing twin babies. Fraternal twins are the most common type of twins. The babies may look similar or different with a different sex in fraternal twins. Identical twins happen when the zygote grows into two individuals. Conjoined twins happen when the zygote separates too late or fails to separate perfectly. Semi identical twins occur when two sperm cells and one egg cell are fertilized. 
The mother can detect pregnancy through various methods and technologies. In addition, reproductive technologies are available for the mother and the father to facilitate fertilization and pregnancy. The mother, however, must be very careful taking care of her health and her baby because many negative methods can affect the embryo. Contraception may occur if the mother practices harmful methods 
The male has the testes and penis. The testes, wrapped in the scrotum, produce sperm, which is also called the gamete, and testosterone, which can also affect hair, vocal cords, and muscles. Each testicle produces about four million sperm per hour. The location of the testes is also vital because they must be placed in a location where the temperature is about two degrees colder than the body temperature. Sperm use their tails to move and their head contains the genetic elements. 
The penis, made up of tissue that can expand and contract, swells with blood and becomes erect when sexually stimulated. During intercourse, smooth muscles cause the mature sperm to move through the vas deferens, a long tube beneath the bladder. Then the sperm are mingled with nutrients. This mixture is called semen. This semen creates fluid so that the sperm can swim even outside the body, give necessary nutrients for the sperm cells, and neutralizes acids for the sperm so that the sperm can survive in the female body. The nutrients necessary for sperm include fructose, amino acids, and vitamin C. The semen continues to travel along the urethra and is released through the process called ejaculation. Normal ejaculation contains only one percent of sperm in the body. 
Two ovaries play a major role in female reproduction. They produce the eggs, also known as oocytes, female gametes, and also create estrogen, the female sex hormone. Estrogen, like testosterone, affect female hair, breast development, size of the pelvis, hips, and thighs. The ovaries are positioned in the abdomen. After the oocytes form in the ovaries, they are discharged into the Fallopian tube. When the eggs are traveling through the tube and meet the sperm, they begin to fertilize, once hey reach the uterus, also known as he womb. The womb can stretch and so that the baby can form inside. The cervix is located at the base of the uterus and contains a tiny opening called the external os. The opening contains mucus which serves as a barrier between the uterus and the opening. The cervix is connected to the birth canal called the vagina. The vagina, covered by numerous folded skin called the labia, is a smooth-muscle-walled tube. The vagina serves to connect the uterus to the outside of the body. The vagina stretches in length when giving birth. 
Newly ejaculated sperm cannot fertilize. They must first undergo the process of capacitation. During this process, sperm remove less important plasma proteins, which consist of proteins and plasma membrane lipids. Capacitation also utilizes calcium and decreases pH level. This process takes places after sperm enter the female reproductive tract and takes several hours to complete. Sperm that have successfully undergone the process of capacitation are called hyperactiviated sperm. After the sperm are ejaculated, few live long enough and surround the egg in the Fallopian tube of the female uterus. 
The Acrosome Reaction
After sperm bind to zona pellucida, the membrane that surrounds the plasma membrane of the egg, they must next penetrate the pellucida to reach the oocyte. The acrosome reaction helps the sperm penetrate the zona pellucida. Membrane fusion and vesiculation cause the sperm’s head to release acrosomal enzymes. The enzymes are located in the head of the sperm, surrounded by the plasma membrane. As the acrosomal enzymes leak through the membrane, the plasma membrane disappears. As the reaction continues and the sperm penetrates the pellucida, the sperm’s head further releases the acrosomal substances. The plasma membrane of the head completely disappears after the sperm crosses the zona pellucida. The sperm use their tails for mobility through the pellucida. However, sperm that lose all their acrosomal substances before reaching the oocyte are useless and cannot fertilize. 
When a sperm cell penetrates the zona pellucida with acrosomal enzymes in its head, the binding and fusion of the plasma membrane and the membrane of the oocyte begin to take place. This unity occurs at the posterior region of the sperm head. Fertilin, a dimeric sperm glycoprotein, then binds to a protein in the oocyte membrane and also have the ability to fuse with the membrane. 
Egg Activation and the Cortical Reaction
Meanwhile, the egg is in the metaphase of the second meiotic division, and thus in a dormant state. When a sperm binds to the membrane of the oocyte, egg activation occurs, during which metabolic as well as physical changes of the egg transpire. The activation also causes the calcium level to increase, and the second meiotic division and the cortical reaction to complete. The cortical reaction causes the egg to release cortical granules shortly after the sperm-oocyte fusion, and fuse with the oocyte plasma membrane. The granules are made up of a mixture of enzymes and proteases. These proteases enter the zona pellucida after the release of the substances from the egg. The proteases, after entering the pellucida, also stimulate the zona reaction. This reaction somehow alters the zona structure. 
After the sperm penetrates the egg, the egg changes its electrical charge. This change stimulates the cortical granule, located beneath the membrane. Other unnecessary sperm perish in about 48 hours. 
The Zona Reaction
The zona reaction is the result of the change in the zona structure performed by proteases. The zona reaction creates protection for an egg that has been fertilized by more than one sperm, also known as polyspermy, which causes the inevitable, immediate death of the embryo. Therefore, the zona reaction sets a wall for the zona pellucida. The second sperm that attempts to enter the pellucida is halted by this wall. This reaction also causes the sperm receptors in the zona pellucida to be destroyed so that no other sperm will be fertilized. 
After a sperm successfully enters the egg, the sperm head fuses with the egg cytoplasm. Then the sperm’s nuclear envelope disappears, and the process of decondensation occurs. During decondensation, the firmly packed chromatin begins to become loosened. Chromatin of both the sperm and the egg are fused into a nuclear membrane, developing into so-called pronuclei. Each of them holds a haploid genome. Eventually, the membranes of the pronuclei collapses, enabling the two genomes to form into chromosomes. When the two pronuclei encounter, the cell division begins. This division results into a diploid organism. Vertebrate fertilization involves the combination of two haploid gametes to form a diploid cell. 
The fertilized egg, zygote, is then pushed along the Fallopian tube. The zygote contains around hundred cells about four days after successful fertilization and cell division. At this stage, the zygote is called a blastocyst because it has around hundred cells now. The blastocyst comes to the uterine lining and it floats for two days and fixes itself in the uterine wall. Pregnancy starts when the blastocyst plants itself in the wall. The blastocyst continues to grow for around nine months. The uterus can expand in size to the size of a basketball. 
Pregnancy can be detected in various ways. It is vital to discern the pregnancy from other conditions or diseases. It is probable that a woman is pregnant under these conditions:
- Ending of period, also known as amenorrhea.
- Breast changes
- Vaginal discoloration
- Changes in skin pigmentation
- Morning sickness, including vomiting and nausea
- Detecting or feeling fetal movements
- Excessive urinary rates
- Abdominal enlargements
- Changes of shape, or size of any sort in uterine and cervical.
- Positive hormonal tests
Pregnancy is deemed positive if:
- Fetal heart tones are felt, heard, or recorded
- A doctor detects fetal movements
- Fetus pregnancy is detected through ultrasound or radiation
The ultrasound test can detect pregnancy at around twenty weeks and can also discover the sex of the baby. In fact, the sex is determined right when the sperm and the egg fuse, but for humans, it is impossible to detect it until around twenty weeks. Fertilized embryo both obtains Wolffian, male sex organs, and Mullerian, female sex organs, ducts. Y chromosome, male hormone testosterone, and anti-Mullerian hormone, or AMH, can affect the sex of the baby. When at about eight weeks, with the development of the internal genitalia, the embryo acquires both an X and a Y chromosome, then the embryo will have the Wolffian duct. If the embryo has two X chromosomes with no Y chromosome, then the embryo will adopt the Mullerian duct and develop into a female. 
In vitro fertilization
Louise Brown became the one to invent in vitro fertilization, IVF. The process involves:
- Obtain mature eggs from the mother after she receives reproductive treatments which help her produce eggs. The eggs, however, need to be eliminated eventually from her ovaries.
- Obtain mature sperm cells from the father.
- Place and mix the eggs and the sperm in a cultural vessel also known as in vitro.
- Develop the fertilized eggs.
- Apply more than two fertilized eggs into the mother’s uterus.
Intracytoplasmic sperm injection
Intracytoplasmic sperm injection, abbreviated as ICSI, involves placing the sperm into the egg. The sperm usually has no problem being fertilized with the egg; however, sperms cannot fertilize. Therefore, the doctor, through this technology, injects the sperm directly into the egg to help fertilization. 
In other cases, the mother’s egg may become defected. In the ooplasmic transfer, the donor egg’s cytoplasm is removed and injected with a sperm into the recipient’s egg. Few successful cases of the ooplasmic transfer have been recorded in animals. However, the use in the United States is prohibited because this process may result in an egg with the donor and the recipient’s mitochondria. This is called the heteroplasmy. This may kill the single sperm injected in the egg. Ultimately, having multiple mitochondria may produce a child with two different genomes of mitochondrial DNA. 
Benefits from the reproductive technologies
- More than four million couples succeeded to have children.
- Prevent pregnancy if the genetic disorders are found.
- Extra fertilized eggs and other fertilizers can be stored for later use. 
Problem concerning these technologies
- The success rate is still low: it is around thirty five percent.
- The risk of having multiple births. Furthermore, these babies may be too light or born too early.
- The possibility of birth defects increases from four to eight percent.
- The risk of causing a genetic problem in the embryo.
Following methods can prevent reproduction.
- Birth control pills
- Surgical procedures that prevent the women and men from producing substances.
- Avoiding sexual activity when the uterus is in reproducing stage.
Diseases can also halt women from having a child. These diseases and infections include:
- STD, sexually transmitted disease
- Genital herpes
In order to avoid these infections, men and women must engage in safe sexual activity. Women’s pregnancy also depends on:
- Hormonal balance
- Tobacco use
- Alcohol use
- Autoimmune disorder
Male reproduction is affected by the following:
- Poor diet
- Tobacco use
- Alcohol use
- Very hot water
The Bible urges that the Christians avoid sexual immorality and live a holy life.
- 1 Corinthians 6:18-20 - "Flee from sexual immorality. All other sins a person commits are outside the body, but whoever sins sexually, sins against their own body. Do you know that your bodies are temples of the Holy Spirit, who is in you, whom you have received from God? You are not your own; you were brought at a price. Therefore honor God with your bodies."
- Genesis 38:27-30 - "When the time came for her to give birth, there were twin boys in her womb. As she was giving birth, one of them put out his hand; so the midwife took a scarlet thread and tied it on his wrist and said, 'This one came out first.' But when he drew back his hand, his brother came out, and she said, 'So this is how you have broken out!' And he was named Perez. Then his brother, who had the scarlet thread on his wrist, came out. And he was named Zerah." 
Fraternal twins are the most common type of twins. The process begins when two eggs from the mother and two sperm cells from the father are fertilized. The pairs then are fertilized and eventually develop into two individual beings. This is also known as dizygotic or biovular twins. Fraternal twins may look very similar or very different. The babies may have a different sex as well. Therefore, fraternal twins can also be described as siblings with the same age. The possibility is more likely if there is a record of conceiving a twin in the family history. The use of pregnancy aid drugs also raises the possibility. Twin pregnancy takes place about one in 60 pregnancies and one in four with reproduction aid drugs. 
Identical twins take place when one egg cell from the mother and one sperm cell from the father are fertilized and the embryo develops into two different individuals in the growth. The separation of the zygote may take place as early as the first two days of fertilization. A type of identical twins, dichorionic, diamniotic twins happens when the zygote have different placentas and sacs. The possibility is about twenty to thirty percent for having identical twins. In monochorionic, diamniotic twins, the zygote divide and while they share placenta, they have different sacs. Monochorionic, diamniotic twins result in very similar genes. Lastly, in monochorionic, monoamniotic twins, the zygote shares the same placenta and sacs. The possibility, though, is approximately one percent of the identical twins. In order this to take place, the zygote is required to separate late enough to share the same placenta and sacs. 
Conjoined twins, also called Siamese twins, occur if the zygote separates extremely late and fails to split completely. However, the possibility is very low. Conjoined twins take place in one in 100,000 child births or one in two hundred MZ twins. These twins share the same placenta, sacs, membranes, body parts, and organs. 
Semi identical twins
If two sperm cells and one egg cell are fertilized, semi identical twins are born. This type of fertilization forms two embryos. However, semi identical twins do not happen easily because more than two sperm cells fertilizing with one egg can be fatal. There have been few cases known of this type. Occasionally, the baby can end up possessing both the male and the female gonads. 
There are two types of fertilization through which organisms reproduce: external and internal fertilization. In external fertilization, sperm and egg cells are both released into the water and the sperm are carried by the current to the eggs. Usually aquatic organisms, such as most fish and amphibians, reproduce through external fertilization. The flagellum enables the sperm also to swim to the egg. One of the important differences in eggs is that in external fertilizing eggs, they do not possess a coat or shell so that sperm can easily enter to be fertilized. Because it is extremely difficult for the sperm to find the egg, the organisms usually release immense number of sperm cells. 
Most land vertebrate and invertebrate reproduce through internal fertilization. In the process, sperm is injected into the female organs and swim with its flagellum to be fertilized with the egg, which is covered with a protective coat. The female organism then keeps the fertilized egg until the embryonic stages pass or release the eggs when they are fertilized. 
The process of fertilization
- Bowen, R. Fertilization Fertilization and Early Embryonic Development. Web. 18 Jan,. 2012.
- Baby2See.comTwins Identical and Fraternal Baby 2 See. Web. 30 January, 2012.
- Hatasaka, H. Harry. Fertilization, Early Pregnancy and Its Disorders Human Reproduction. Web. 18 Jan. 2012.
- Mark 10:6-12 (New International Version) Biblegatway. Web. 31 Jan. 2012.
- Freudenrich, Craig and Edmonds, Molly How Human Reproduction Works HowStuffWorks. Web. 18 Jan. 2012.
- Sexual Reproduction in Humans Sexual Reproduction in Humans. Web. 5 December, 2011.
- 1 Corinthians 6:18-20 (New International Version) BibleGateway.com. Web. 18 Jan. 2012.
- Genesis 38:27-30 (New International Version) Biblegateway. Web. 30 January, 2012.
- Campbell, Darlene. External vs. Internal Fertilization Cornell University Blog 1105-1106. Web. 30 January, 2012.