Prenatal Development and Birth – Chapter 3
Stages of Prenatal Development
Conception takes place during a woman's ovulation or within a few days of it; the
ovum, or egg, once released from the mother's ovaries, lives only about three to
five days. Prenatal development then encompasses the 38 weeks or
approximately 9 months, between conception and birth. Traditionally, pregnancy
has been described as occurring in three trimesters, three periods of three months
each. Increasingly however, we talk about the three periods of:
1) The zygote: The developing organism from the time of the union of the
sperm and the egg to about the second week of gestation; the period of the
zygote is comprised of the implantation of the fertilized egg in the wall of
the uterus. When this occurs, about one week after conception, the zygote
is very small. At this point the zygote forms the physiological dependence
with the mother that will continue throughout the course of prenatal
2) The Embryo: This is a period of rapid growth that lasts from the beginning
of the third week of gestation (the carrying of an embryo or fetus during
pregnancy, usually for nine months in humans) until the end of the eighth
week. During this period the embryo becomes recognizable as a tiny
human. During this period the infant increases 2 million percent in size.
During this period three crucial structures develop to support the embryo:
1. The amniotic sac: a membrane containing a watery fluid that encloses
the developing organism, protecting it from physical shocks and
2. The placenta: A fleshy, disc-like structure formed by cells from the
lining of the uterus and from the zygote, and that, together with the
umbilical cord, serves to protect and sustain the life of the growing
3. Umbilical cord: A tube that contains blood vessels that carry blood back
and forth between the growing organism and its mother by way of the
placenta; it carries oxygen and nutrients to the growing infant and
removes carbon dioxide and waste products.
During the embryonic period, the inner mass of the developing organism
differentiates into three layers:
1. The ectoderm: The hair, nails, parts of the teeth, outer layer of the skin,
skin glands, the sensory cells and the nervous system develop from this.
2. The mesoderm: Forms into the muscles, skeleton, circulatory and
excretory systems, and the inner skin layer.
3. The endoderm: Forms into the gastrointestinal tract, trachea, bronchia,
Eustachian tubes, glands, and vital organs, such as the lungs, pancreas,
Prenatal development is guided by two principles:
1. Cephalocaudal: The pattern of human physical growth in which
development begins in the area of the brain and proceeds downward, to
the trunk and legs.
2. Proximal-distal: The pattern of human physical growth wherein
development starts in central areas, such as the internal organs, and proceeds to more distant areas, such as arms and legs.
3) The Fetus: The term for the developing organism from the third month of
gestation to delivery. During this fetal period the development of bodily
structures and systems becomes complete. At the end of the third month
the fetus has all of its body parts, by the end of the fourth month mothers
usually report movement of the fetus. At around five months, reflexes such
as sucking, swallowing and hiccupping, usually appear. After the fifth month
the fetus develops nails, and sweat glands, coarser, more adult-like skin and
soft hair called lanugo (soft hair that covers the fetus's body from about
the fifth month of gestation on; may be shed before birth or after). By six
month, the eyes can open and close.
- Respiratory distress syndrome: A condition of the newborn marked by
laboured breathing and a bluish discolouration of the skin or mucous membranes,
and which often leads to death. Usually occurs in children that are born
prematurely at six months because their regulatory processes are not mature
enough, and cannot produce or maintain an adequate amount of surfactant (a
liquid that allows the lungs to transmit oxygen from air to blood).
- The age of viability: The age of 22 to 26 weeks, by which point the fetus's
physical systems are well enough advanced that it has a chance at survival if born
prematurely. However, babies born before 28 weeks can still have many
difficulties, since many systems are still developing.
Risks in Prenatal Environment
Teratogen: An environmental agent, such as a drug, medication, dietary
imbalance, or polluting substance that may cause developmental deviations in a
growing human organism; most threatening in the embryonic stage but capable of
causing abnormalities in the fetal stage as well. Teratogens exert their affects on
prenatal development in specific ways:
1. A teratogen exerts its effects largely during critical periods.
2. Each teratogen exerts certain specific effects.
3. Either maternal or fetal genotypes may counteract a teratogen's effects.
4. The effects of one teratogen may intensify the effects of another.
5. Different teratogens may produce the same defect.
6. The longer a fetus is exposed to a particular teratogen and the greater the
intensity of the teratogen's effects, the more likely it is that the fetus will be
Nicotine and alcohol
Sudden infant death syndrome (SIDS): Infants under the age of six months
stop breathing and die without apparent cause. This syndrome is more common in
the offspring of mothers who smoke, drink, or take narcotic drugs.
Fetal alcohol syndrome (FAS): A disorder exhibited by infants of alcoholic
mothers and characterized by stunted growth, a number of physical and
physiological abnormalities, and often, mental retardation. The fetal damage from
alcohol appears to be the greatest in the last trimester. Fetal alcohol spectrum disorder (FASD): An umbrella term used to describe
the range of effected associated with prenatal exposure to alcohol. Unfortunately
it is not yet known how much or how little alcohol consumption leads to problems
Heroin, Cocaine, and Other Drugs
Mother who are addicted to heroin or use cocaine have offspring who are also
addicted or sustain toxic effects from these drugs. Babies addicted to one of these
drugs go through withdrawal symptoms. In some, cases the symptoms can be
severe enough to result in infant's death. Although not all mothers that take
cocaine have babies with developmental anomalies, cocaine use is related to a
series of physical defects in infants, including bone, genital, urinary tract, kidney,
eye and heart deformities, and brain hemorrhages.
− Lead: Exposure during pregnancy has been associated with a variety of
problems in newborns, including prematurity and low birthweight, brain
damage, and physical defects, as well as with long-term problems in
cognitive and intellectual functioning.
− Polychlorinated biphenyls (PCBs): Were routinely used in electrical
transformer and capacitators until they were banned in the mid 70s. Women
who PCB contaminated fish gave birth to infants that were smaller, less
responsive, and less neurologically advanced.
− Father's exposure to environmental toxins can affect their sperms, and this
ultimately have harmful effects on the developing fetus. The exposure may
lead to chromosomal abnormalities that may affect their fertility or may
increase the risk of their pregnant wives miscarrying or bearing infants with
Medical Interventions in Pregnancy and Children
Physicians may prescribe drugs or diagnostic procedures to alleviate discomfort
during pregnancy, sometimes these drugs too have proven dangerous.
− Diethylstilbestrol (DES): A synthetic hormone once prescribed to
pregnant women to prevent, miscarriages but discontinued when cancer
and precancerous conditions were detected in the children of such women.
− Thalidomide: A drug once prescribed to relieve morning sickness in
pregnant women but discontinued when found to cause serious
malformations of the fetus. Current controversy surrounds possible use in
treating symptoms of such diseases as AIDS, cancer, and leprosy.
Medications used in labour and delivery
Babies of mothers who received large amounts of obstetrical medication during
labour showed less responsiveness, less smiling, and more irritability for several
days after birth, as ell as depression, motoric disorganization, and disruptions in
feeding responses. They also have impaired attention and motor abilities at 1
month of age but not longer. Maternal Factors
Some factors affecting the fetus are directly related to characteristics of the
− Age and parity (the number of children she has already borne): A
woman's age and parity may interact in influencing the development of her
fetus. Women who have their first child when they are under 15 or over 35
years are likely to experience more problems during pregnancy and
complications during delivery than other women. The chance of becoming
pregnant declines s