Once the zygote implants in the uterine wall, embryonic and fetal development continue through three trimesters to birth.
Describe the development of the human fetus from fertilization through the third trimester
- After fertilization, the zygote implants itself in the uterine wall; its outer layer grows into the endometrium, where it begins to produce human chorionic gonadotropin.
- During the first trimester, the placenta forms along with the internal organs and structures; however, not all of the internal organs function at this point.
- During the second trimester, internal organs continue to develop and the fetus becomes active.
- The third trimester is one of rapid growth, in which the fetus reaches its full size; pregnancy often becomes uncomfortable for the mother.
- zygote: a diploid fertilized egg cell
- chorion: allows exchange of oxygen and carbon dioxide between the embryo and the egg’s external environment
- human chorionic gonadotropin: a peptide hormone, produced during pregnancy, that prevents the breakdown of the corpus luteum and maintains progesterone production
- placenta: a vascular organ in mammals that supplies food and oxygen from the mother to the fetus, while passing back waste; it is implanted in the wall of the uterus
Twenty-four hours before fertilization, the egg has finished meiosis and become a mature oocyte. When fertilized (at conception), the egg, now known as a zygote, travels through the oviduct to the uterus. The developing embryo must implant into the wall of the uterus within seven days or it will deteriorate and die. The outer layers of the zygote ( blastocyst ) grow into the endometrium by digesting the endometrial cells. Wound healing of the endometrium closes up the blastocyst into the tissue. Another layer of the blastocyst, the chorion, begins releasing a hormone called human chorionic gonadotropin (hCG) which makes its way to the corpus luteum, keeping it active. This ensures adequate levels of progesterone that will maintain the endometrium of the uterus for the support of the developing embryo. Pregnancy tests determine the level of hCG in urine or serum: if the hormone is present, the test is positive.
The gestation period is divided into three equal periods or trimesters. During the first two to four weeks of the first trimester, nutrition and waste are handled by the endometrial lining through diffusion. As the trimester progresses, the outer layer of the embryo begins to merge with the endometrium and the placenta forms. This organ takes over the nutrient and waste requirements of the embryo and fetus, with the mother’s blood passing nutrients to the placenta and removing waste from it. Chemicals from the fetus, such as bilirubin, are processed by the mother’s liver for elimination. Some of the mother’s immunoglobulins will pass through the placenta, providing passive immunity against some potential infections.
Internal organs and body structures begin to develop during the first trimester. By five weeks, limb buds, eyes, the heart, and liver have been basically formed. By eight weeks, the term fetus applies; the body is essentially formed. The individual is about five centimeters (two inches) in length and many of the organs, such as the lungs and liver, are not yet functioning. Exposure to any toxins is especially dangerous during the first trimester, as all of the body’s organs and structures are going through initial development. Anything that affects that development can have a severe effect on the fetus’ survival.
During the second trimester, the fetus grows to about 30 cm (12 inches). As it becomes active, the mother usually feels the first movements. All organs and structures continue to develop. The placenta has taken over the functions of nutrition and waste, along with the production of estrogen and progesterone from the corpus luteum, which has degenerated. The placenta will continue functioning up through the delivery of the fetus.
During the third trimester, the fetus grows to 3 to 4 kg (6 ½ -8 ½ lbs.) and about 50 cm (19-20 inches) long. This is the period of the most rapid growth during the pregnancy. Organ development continues to birth (and some systems, such as the nervous system and liver, continue to develop after birth). The mother will be at her most uncomfortable during this trimester. She may urinate frequently due to pressure on the bladder from the fetus. There may also be intestinal blockage and circulatory problems, especially in her legs. Clots may form in her legs due to pressure from the fetus on returning veins as they enter the abdominal cavity.
Labor and Birth
Labor and birth are divided into three stages: the dilation of the cervix, the delivery of the baby, and the expulsion of the placenta.
Describe the process of labor and birth in humans
- At the end of gestation, estrogen receptors on the uterine wall bind oxytocin, which causes the uterine muscles to contract; as the muscles contract, they signal for the release of more oxytocin in a positive feedback loop.
- During the first stage of labor, the cervix thins and dilates to allow passage of the baby into the birth canal; typically over the course of several hours, the cervix will dilate to its full width of 10 centimeters.
- During the second stage of labor, contractions become very strong and, aided by the pushing of the mother, the baby is expelled from the uterus.
- During the third stage of labor, the placenta, amniotic sac, and the remainder of the umbilical cord are expelled from the uterus, usually within a few minutes after birth.
- When the baby begins suckling at the breast after delivery, prolactin signals the release of milk from the mammary glands, providing nutrition and immunity against disease to the infant.
- prolactin: a peptide gonadotrophic hormone secreted by the pituitary gland; it stimulates growth of the mammary glands and lactation in females
- parturition: the act of giving birth; childbirth
- oxytocin: a hormone that stimulates contractions during labor, and then the production of milk
Labor and Birth
Labor is the physical effort of expulsion of the fetus and the placenta from the uterus during birth (parturition). The total gestation period from fertilization to birth is about 38 weeks (birth usually occurring 40 weeks after the last menstrual period). Toward the end of the third trimester, estrogen causes receptors on the uterine wall to develop and bind the hormone oxytocin. At this time, the baby reorients, facing forward and down with the back or crown of the head engaging the cervix (uterine opening). This causes the cervix to stretch, sending nerve impulses to the hypothalamus, which signals for the release of oxytocin from the posterior pituitary. The oxytocin causes the smooth muscle in the uterine wall to contract. At the same time, the placenta releases prostaglandins into the uterus, increasing the contractions. A positive feedback relay occurs between the uterus, hypothalamus, and the posterior pituitary to assure an adequate supply of oxytocin. As more smooth muscle cells are recruited, the contractions increase in intensity and force.
There are three stages to labor. During stage one, the cervix thins and dilates. This is necessary for the baby and placenta to be expelled during birth. The cervix will eventually dilate to about 10 cm, a process that may take many hours, especially in a woman bearing her first child. At some point, the amniotic sac bursts and the amniotic fluid escapes. During stage two, the baby is expelled from the uterus with the umbilical cord still attached. The uterus contracts and the mother pushes as she compresses her abdominal muscles to aid the delivery. The last stage is the passage of the placenta after the baby has been born and the organ has completely disengaged from the uterine wall, usually within a few minutes. If labor should stop before stage two is reached, synthetic oxytocin, known as Pitocin, can be administered to restart and maintain labor.
The mother’s mammary glands go through changes during the third trimester to prepare for lactation and breastfeeding. When the baby begins suckling at the breast, signals are sent to the hypothalamus causing the release of prolactin from the anterior pituitary, which signals the mammary glands to produce milk. Oxytocin is also released, promoting the release of the milk. The milk contains nutrients for the baby’s development and growth as well as immunoglobulins to protect the child from bacterial and viral infections.
Contraception and Birth Control
Contraception, also known as birth control, is methods used to prevent pregnancy; some of these methods are more successful than others.
Describe the various methods of birth control and their associated failure rates
- Barrier methods include those that prevent the sperm from reaching the egg ( condoms, diaphragms, cervical caps, sponges, and spermicides ); these methods have a failure rate of 15-24%.
- Hormonal methods include those that prevent a woman from ovulating (oral birth control pills, hormone injections, and hormone implants); these have a failure rate of 8%.
- Sterilization, such as a tubal ligation in a woman or a vasectomy in a man, is an extremely effective (although permanent) method of contraception, with a failure rate of 1%.
- More “natural” methods of birth control include avoiding intercourse when a woman is ovulating, or withdrawing the penis from the vagina before ejaculation; these methods are much less successful, with a failure rate of around 25%.
- condom: a flexible sleeve made of latex or other impermeable material such as sheepskin, worn over an erect penis during intercourse as a contraceptive or as a way to prevent the spread of STDs
- intrauterine device: a contraceptive device consisting of a spiral or similar shape of plastic or metal inserted through the vagina into the uterus in order to prevent the implantation of a fertilized egg
- vasectomy: the surgical removal of all or part of the vas deferens, usually as a means of male sterilization
- contraception: the use of a device or procedure to prevent conception as a result of sexual activity
- spermicide: a substance used for killing sperm
Contraception and Birth Control
Contraception, also known as birth control, is methods or devices used to prevent pregnancy. Strictly speaking, contraception aims to prevent the sperm and egg from joining, while birth control can refer to methods used to prevent a fertilized egg from developing into a fetus. Both terms are, however, frequently used interchangeably.
There are many methods of birth control, including barriers to sperm, hormones that prevent ovulation, sterilization procedures, and “natural” methods. Each method has an associated “failure rate”, which is the number of pregnancies resulting from the method’s use over a twelve-month period.
Barrier methods include those that prevent the sperm from reaching the egg. These include condoms, diaphragms, cervical caps, sponges, and spermicides; they have a failure rate of 15-24%. Barrier methods such as condoms, cervical caps, and diaphragms serve to block sperm from entering the uterus, thereby preventing fertilization. Chemicals such as spermicides, which are designed to kill sperm, are often used in conjunction; sponges, for example, are saturated with spermicides and are placed in the vagina at the cervical opening. Combinations of spermicidal chemicals and barrier methods achieve lower failure rates than do the methods when used separately.
Hormonal methods use synthetic progesterone (sometimes in combination with estrogen) to inhibit the hypothalamus from releasing FSH or LH, preventing an egg from being available for fertilization. The method of administering the hormone affects failure rate, although, in general, hormonal methods have a failure rate of 8%. The most reliable method, with a failure rate of less than 1 percent, is the implantation of the hormone under the skin. The same rate can be achieved through the sterilization procedures of vasectomy in the man or of tubal ligation in the woman, or by using an intrauterine device (IUD). IUDs are inserted into the uterus where they establish an inflammatory condition that prevents fertilized eggs from implanting into the uterine wall.
Sterilization is a one-time, permanent, surgical procedure. In a vasectomy, the vasa deferentia of a male are severed and then tied/sealed in a manner that prevents sperm from entering into the seminal stream (ejaculate). Tubal ligation or tubectomy is a surgical procedure for sterilization in which a woman’s fallopian tubes are clamped and blocked, or severed and sealed; either method prevents eggs from reaching the uterus for fertilization. Both of these procedures have a less than 1% failure rate.
Natural methods include avoiding intercourse when ovulation is occurring (“natural family planning) or withdrawing the penis from the vagina before ejaculation. Nearly 25% of the couples using natural family planning or withdrawal can expect a failure of the method. Natural family planning is based on the monitoring of the menstrual cycle and having intercourse only during times when the egg is not available. A woman’s body temperature may rise a degree Celsius at ovulation and the cervical mucus may increase in volume, becoming more pliable. These changes give a general indication of when intercourse is more or less likely to result in fertilization. Withdrawal involves the removal of the penis from the vagina during intercourse, before ejaculation occurs; it has a failure rate of 27%. This is, therefore, a risky method. The high failure rate is due to the possible presence of sperm in the bulbourethral gland’s secretion, which may enter the vagina prior to removing the penis.
Termination of an existing pregnancy can be spontaneous or voluntary. Spontaneous termination in the first trimester is referred to as spontaneous abortion, while in the second and third trimesters is referred to as a miscarriage. Spontaneous abortion usually occurs very early in the pregnancy (typically within the first few weeks). Pregnancy loss occurs when the embryo/fetus cannot develop properly so the gestation is naturally terminated. Voluntary termination of a pregnancy is called an elective abortion. Laws regulating elective abortion vary between states; the laws tend to view fetal viability as the criteria for allowing or preventing the procedure.
Infertility, or the ability to conceive, can be treated by assisted reproductive technologies which can aid conception.
Define infertility and discuss ways in which it can be treated
- Infertility can have many causes, such as sexually transmitted diseases, toxins, malnutrition, drug use, stress, and age.
- Infertility can be treated by assisted reproductive technologies in which science intervenes, attempting to aid conception through artificial means.
- In vitro fertilization is a type of assisted reproductive technology in which an egg is fertilized by a sperm outside the body, grows to the eight- cell stage, and is then implanted in the uterus.
- infertility: the inability to conceive children
- assisted reproductive technology: the technological means of inducing pregnancy (by means of artificial insemination or other techniques)
Infertility is the inability to conceive or carry a fetus to birth. About 75 percent of causes of infertility can be identified. These include diseases (such as sexually-transmitted diseases that can cause scarring of the reproductive tubes in either men or women) or developmental problems frequently related to abnormal hormone levels in one of the individuals. Inadequate nutrition, especially starvation, can delay menstruation. Stress can also lead to infertility. Short-term stress can affect hormone levels, while long-term stress can delay puberty, causing less-frequent menstrual cycles. Other factors that affect fertility include toxins (such as cadmium), tobacco smoking, marijuana use, gonadal injuries, and aging.
If the cause of infertility is identified, several assisted reproductive technologies (ART) are available to aid conception. A common type of assisted reproductive technology is in vitro fertilization (IVF) where an egg and sperm are combined outside the body and then placed in the uterus. Eggs are obtained from the woman after extensive hormonal treatments that prepare mature eggs for fertilization and prepare the uterus for implantation of the fertilized egg. Sperm are obtained from the male and combined with the eggs, which are then supported through several cell divisions to ensure viability of the zygotes. When the embryos have reached the eight-cell stage, one or more is implanted into the woman’s uterus. If fertilization is not accomplished by simple IVF, a procedure known as intracytoplasmic sperm injection (ICSI) can be used to inject the sperm into an egg. IVF procedures produce a surplus of fertilized eggs and embryos that can be frozen and stored for future use; the procedures can also result in multiple births.