Prenatal Development

Chapter 3

Key Chapter Questions

How does life begin?

What happens in the womb before birth?

How do a pregnant woman’s health habits and environment affect the developing embryo and fetus?

What is considered a healthy pregnancy?

What is the role of social institutions in supporting pregnancy and parenting?

Conception

Meiosis – The process of cell division in which gametes are formed; it results in four new cells, each containing 23 single chromosomes, each different from the original cell and all different from one another.

Gametes – sexual reproduction cells Ova—female reproductive cell Sperm—male reproductive cell

Fertilization

Fertilization—process that occurs in the fallopian tubes when the sperm and ova combine to form a zygote

Zygote—a single cell organism that results from the fertilization of an egg cell by a sperm; the fertilized egg

Vocabulary

Prenatal Growth and Development

The Germinal Period: 0 – 2 Weeks

Embryo—the term given to a developing human organism between the first cleavage of the single-cell zygote into multiple cells and 8 weeks postfertilization

Blastocyst—the term given to the embryo at about day 5 or 6 postfertilization

Embryoblast – the inner cell mass of the blastocyst, which is the developing human organism

Amniotic sac – a thin membrane arising from a cell mass within the blastocyst that completely surrounds the embryo/fetus and contains a protective fluid in which the embryo/fetus is immersed

Trophoblast – a cell mass within the blastocyst that becomes the placenta

Germinal Period

Differentiation of the Embryonic Germ Layers

Germinal period – the first two weeks of prenatal development from fertilization of the egg to the separation of the three germ layers of somatic cells, and including implantation of the blastocyst of the embryo into the uterine wall Ectoderm Endoderm Mesoderm

Ectoderm – the outer germ layer of the blastocyst that develops into the nervous system (including the brain and spine), skin, nails, and hair, as well as the salivary, pituitary, and mammary glands

Endoderm – the innermost germ layer of the blastocyst that becomes the thyroid gland, bladder, lungs, and digestive system

Mesoderm – the germ layer between the ectoderm and endoderm that becomes the heart, circulatory and lymph systems, connective tissue, muscle, and bones

Layers

Development and Attachment

The Period of the Embryo: 3 to 8 Weeks

Embryonic period – the period of prenatal development starting at the beginning of week 3 and continuing through week 8; organogenesis takes place

Organogenesis – a period of development of major organ systems during the embryonic period; by the end of week 8, 95% of major organs are complete

A structure formed when the trophoblast burrows into the lining of the uterus, joining the uterine mucous membrane with the membranes of the fetus. Substances ingested by the mother cross the placenta via the umbilical cord to the developing fetus, providing nourishment for its development.

Placenta

Third week: The neural plate forms and becomes the brain and neural tube (spinal cord). Neurogenesis, the production of neurons begins. The heart chambers and blood vessels develop.

Fourth week: The heart begins to beat, arm and leg buds are visible, eyes, ears, nerves, and muscular, skeletal, and digestive systems begin to form. Vertebrae are present, major veins and arteries are completed.

Fifth week: The brain has five components and the nose and lips begin to form.

Embryonic Period

Sixth week: Differentiation of external genitalia and internal reproductive organs begins. The head and brain are prominent; arms and legs are longer; and hands and feet have fingers and toes. The lungs begin to develop.

Seventh week: The face, eyelids, and neck begin to form. The stomach and other internal organs are in position. Muscles are forming, arms and legs are apparent, and the embryo can move.

Eighth week: The embryo appears more human, the inner and middle ear develop, and the embryo is the size of a grape.

The Amnion

• The amnion is one of the life support systems that develops from the trophoblast

• Develops from the fertilized egg not from the mothers body

The Period of the Fetus: 9 Weeks to Birth

The period of prenatal development that begins after organogenesis is 95% complete at about week 9 and continues until birth. 9 to 12 weeks: Intestines are in position, the spinal cord

is visible, the eyes take final form and the eyelids can close, red blood cells form in the liver. Sex organs are now apparent. Toothbuds appear. The heartbeat is audible with the right equipment. The fetus is 2.4 to 3 inches long and weighs 0.7 oz.

13 to 16 weeks: The skin is almost transparent, the bones and joints are distinct, the brain’s hemispheres are visible. Lanugo (fine hair) and vernix caseosa (oil) begin to appear on skin. The fetus is 4.7 inches long and weighs 3.5 oz. The mother may feel quickening (fetal movement).

17 – 20 weeks: Dental enamel forms . Myelination of nerves begins. The intestines and kidneys work. Ultrasound is typically done at around 20 weeks.

By 24 weeks: Fat begins to accumulate. Bone marrow begins producing red blood cells. Eyes are fully formed. The fetus is 9 inches long and weighs 21.6 oz.

Fetal Development from Week 25 to Week 40

Viability – The ability of the fetus to survive outside the womb; at about week 24 to 25. By 28 weeks: The brain continues to develop, fetal activity

increases, fat forms, fingernails appear, the eyes open, close, and blink in response to light and sound. The fetus is 10.6 inches long and weighs 38 oz.

By 32 weeks: The fetus responds to sound and external pressure, in males, the testes descend, the brain is 25% of adult weight. The fetus is 12.2 inches long and weighs 4 to 4.6 lbs.

By 36 weeks: The lanugo begins to disappear, physical growth slows while the brain continues developing and becomes more convoluted, and the fetus has obvious sleep and wake cycles.

By 40 weeks: The fetus gains 50% of its birth weight in the last month. The skin is smooth. The head has hair and lanugo only remains on the shoulders. Myelination increases. The fetus is the size of the average newborn: roughly 17 – 20 inches long and weighing about 7 lbs.

The Developing Brain

Neural proliferation and migration: Brain cells are developed (neurogenesis) and then migrate into specific areas of the developing brain.

Neural differentiation and myelination: The migrating brain cells differentiate into specific types of cells in the various parts of the brain and nervous system, and myelin sheaths develop around their axons.

Synaptogenesis and pruning: As the differentiated cells mature, synapses develop between them so that the cells become connected. First, a surplus of cells develops; later, these will be pruned.

Third trimester

Second trimester

First trimester

Anticipates end; uterus expands to point below breastbone

Less fatigue and nausea; uterus expands into abdominal cavity

Very tired; nausea and pregnancy sickness; breasts change; emotional changes