Chapter 4: Prenatal Development

Germinal period or the period of the zygote

From conception to implantation in the uterine wall

Period of the embryo

from week 3 to week 8

Period of the fetus

from week 9 to birth

Support structures develop once

implantation has occurred

Implantation

when the developing cluster of cells attaches to the uterine wall

Amnion

The membrane that surrounds the embryo or fetus and holds the protective amniotic fluid

Chorion

The outer membrane that helps form the placenta

Placenta

An organ that exchanges oxygen, nutrients, and waste between the pregnant person and the developing baby.

Umbilical cord

Connects the fetus to the placenta and carries blood containing oxygen and nutrients to the fetus, while carrying waste away.

Placental exchange

The transfer of oxygen and nutrients to the embryo or fetus and the transfer of waste products away through the placenta.

Maternal and fetal blood

The pregnant person’s blood and the embryo’s or fetus’s blood do not mix directly. Materials pass between them through a thin membrane in the placenta.

Placenta also qualifies as an

endocrine organ

Ectoderm

The outer cell layer, which develops into the nervous system, skin, and hair. Part of it forms the neural tube.

Neural tube

A structure that develops from part of the ectoderm and forms the brain and spinal cord.

Mesoderm

The middle cell layer, which develops into muscles, bones, and the circulatory system.

Endoderm

The inner cell layer, which develops into the digestive system, lungs, urinary tract, and other vital organs.

Embryonic development during the second month

The ears begin to form, the early skeleton appears, the limbs develop, and the brain grows rapidly. Indifferent gonad appears

Neuron production begins around

the fifth week of embryonic development.

Indifferent gonad

An early reproductive structure that has not yet developed into male or female reproductive glands.

Embryo size at the end of the second month

About 2.5 centimeters long and 7 grams in weight.

Fetal development in the third month

The fetus can kick and twist, though these movements usually cannot yet be felt by the pregnant person. It can swallow, digest, and urinate. In male fetuses, the testes begin secreting testosterone. Ultrasound may be able to detect the fetus’s sex. The fetus is about 7.5 centimeters long and weighs under 28 grams.

Fetal development during months 4 through 6

Motor movements become more refined, including thumb-sucking. The pregnant person may begin to feel kicking, and the heartbeat can be heard with a stethoscope. Eyebrows, eyelashes, and scalp hair appear.

Vernix

A white, cheesy coating that covers the fetus’s skin and helps protect it from becoming chapped. Months 4 to 6

Lanugo

A fine layer of hair that covers the fetus’s body. Months 4 to 6

Fetal development during months 7 through 9

The organ systems mature, and the fetus gains weight. This is sometimes called the “finishing phase.”

Age of viability

The point in prenatal development when survival outside the uterus becomes possible. At 22 to 28 weeks.

Fetal position in the ninth month

The fetus usually moves into a head-down position.

Fetus size near birth

About 48 to 53 centimeters long and 3.2 to 3.6 kilograms in weight.

Teratogens

Environmental agents such as viruses, drugs, chemicals, radiation that can harm developing fetus

Environmental factors that affect prenatal development

teratogens, environmental hazards, and maternal characteristics

Each developing structure has

a sensitive period

Sensitive period

a time when it is particularly susceptible to damage from teratogens

Once fully formed, body parts are

less susceptible to damage

Rubella

Most dangerous in first trimester. Effects include blindness, deafness, cardiac abnormalities, intellectual disability

Toxoplasmosis

Caused by a parasite acquired from undercooked meat or handling feces from cats. Effects include brain damage in first trimester; miscarriage thereafter

Syphilis

not dangerous until 18th prenatal week. Effects include eye, ear, bone, heart, defects or brain damage, or miscarriage

Genital herpes

most infectious at birth. Effects include death, blindness, brain damage, neurological disorders

HIV/AIDS

screening and antiretroviral treatment have reduced prenatal transmission to 2% (In NA and Europe). Effects of exposure in developing countries include low birth weight, impaired early growth, impaired psychomotor and cognitive development, immune issues

Thalidomide

A medication once used to relieve morning sickness. When taken early in pregnancy, it caused severe birth defects (eyes, ears, noses, hearts, and phocomelia) in some babies, though not all exposed babies were affected. Animal tests using rats did not reveal the same fetal harm.

Phocomelia

A birth defect in which the arms or legs are severely underdeveloped, sometimes appearing close to the body because the long bones are missing or shortened.

Diethylstilbestrol (DES)

A synthetic form of estrogen that was given to some pregnant women. Effects of exposure before birth were not always apparent until the exposed children reached adulthood.

Fetal Alcohol Syndrome (FAS)

Caused by heavy drinking. Effects include microencephaly, facial malformation, hyperactivity, seizures, below average intelligence

Microencephaly

A condition in which a baby’s head is much smaller than expected, often because the brain did not develop or grow as expected.

Fetal alcohol effects (FAE)

Caused by moderate, social drinking. Effects include poor motor skills, attention problems, below average intelligence.

Cigarette smoking

slows growth of fetus. Associated with low birth weight and increased risk of cleft lip with or without cleft palate. Increased risk of miscarriage or SIDS. Long term effects less clear

Marijuana use during pregnancy

lower birth weight and neuronal abnormalities

Marijuana effects short term

tremors, sleep disturbances, and lack of interest in surroundings in infancy

Marijuana effects long term

10 year olds show deficits in reading and general underachievement as well as more symptoms of anxiety and depression. Young adults still show differences in executive and accompanying neurophysiological functioning

Cocaine

causes premature births, which means smaller babies, lower IQs

Heroin

Increased neonatal death. Infants are born addicted, which means they go through withdrawal after birth

Radiation

High exposure can result in serious physical deformities, intellectual disabilities, and death

Chemicals and pollutants

can cause physical deformities and intellectual disabilities

Maternal Characteristics

Socioeconomic status (SES), diet, emotional well-being, age, Rh factor incompatibility

Rh incompatibility

A blood-type mismatch that can occur when an Rh-negative pregnant person carries an Rh-positive fetus. The pregnant person’s immune system may make antibodies that can harm the fetus’s red blood cells, especially in a later pregnancy.

SES (socioeconomic status)

A person’s social and economic circumstances, including income and education.

SES and infant mortality

Lower income and lower education are related to higher infant mortality.

Recommended pregnancy weight gain

11 to 14 kg recommended

Malnutrition

Inadequate nutrition during pregnancy. It can contribute to smaller babies, disrupt spinal cord development during the first trimester, and lead to low birth weight when it occurs during the third trimester.

Protein-calorie intake

The amount of protein and calories in the diet. Adequate intake is important for brain development.

Folic acid (folate)

A nutrient crucial for neural tube development.

Spina bifida

A birth defect that occurs when the neural tube does not develop or close properly.

Increased stress during pregnancy

associated with decreased birth weight and increased birth complications.

Maternal age

The pregnant person’s age during pregnancy. Risks associated with both teen mothers and older mothers.

Most genetically abnormal embryos

do not develop to term

Healthy newborns

95% of newborns are healthy.