Human Life Stages

Pre - Fertilization

Egg cells are produced in the ovaries

The egg then moves to the oviduct for fertilization.

Fertilization

Fertilization is the uniting of a sperm and an egg, which begins the entire process of growth and development. Once again, this occurs in the oviduct.

Fertilization

When the sperm and egg cells come together, they create a fertilized cell called a zygote.

We all start out as a

SINGLE cell

ZYGOTE

0-12 weeks

The zygote undergoes a series of mitotic divisions known as cleavage which eventually produces a clump of cells called an embryo.

Cleavage continues as the embryo moves toward the uterus. By the time it reaches the uterus, the embryo has grown into a hollow ball of cells called a blastocyst.

First Trimester

Implantation occurs about six days after fertilization when the blastocyst burrows into the lining of the uterus.

Stages of Egg Development

Types of Twins

Multiple births result from multiple egg releases (fraternal twins) or division of zygote (identical twins).

Identical vs Fraternal Twins

Identical Twins Fraternal Twins

Identical vs Fraternal Twins

Identical Twins Fraternal Twins

Conjoined Twins

Conjoined twins will form when the zygote for identical twins fails to separate.

After implementation, the embryo is known as a fetus

This is where the fetus is most susceptible to drugs

First Trimester

2 days

1 week

4 weeks

8 weeks

11 weeks

13 to 28 weeks During the second trimester (4 – 6 months) the

brain develops and begins to control muscle responses, bone development continues, and toenails and fingernails form.

By the end of the fifth month, the fetus begins to move, stretching its arms and legs, and even sucking its thumb.

Second Trimester

13 weeks 20 weeks

26 weeks

28 to 40 weeks

In the third trimester (7 – 9 months) the nerves develop and the fetus can hear, smell, and see light.

By the end of the third trimester, the fetus is able to live outside the mother’s body. The fetus leaves the mother’s body in a process called labor, which usually lasts several hours.

During labor, the walls of the uterus contract and expel the fetus from the uterus through the vagina.

Third Trimester

Umbilical Cord

The umbilical cord connects the embryo to the placenta which provides nourishment from the mother and removes wastes from the embryo.

Amniotic Sac

The amniotic sac cushions the embryo and stores nutrients and wastes.

.

Placenta

Made up of uterine lining - provides nutrients

Placenta

Food & gases diffuse across

blood vessels

Birthing Process

Muscular contractions of the uterus push the baby out through the vagina.

The umbilical cord is cut after the baby’s birth, and the scar that forms is called the navel (belly button).

Babies who cannot be delivered through the birth canal are delivered by caesarian section (C-section).

Stages After Birth

Infancy – about the first 18 months of life. Newborns experience fetal stress as

they adapt to life outside the uterus. First four weeks are known as the

neonatal period. During this period, the infant’s body begins to function normally.

Infants are dependent on humans for their survival.

Infants quickly increase physical coordination, mental development, and size.

Stages After Birth

Childhood – from 18 months to puberty (~12 years old) when physical and mental development continues.

Stages After Birth

Adolescence – begins with puberty (reproductive maturity) and ends about age 16-18; hormones produce secondary sex characteristics.

Stages After Birth

Adulthood – from approximately age 18 and continues through old age. Muscles and skeleton

cease growing. In middle adulthood (30-

60 years) body begins to slowly decline.

In older adulthood (after age 60) physical body systems decline; average life span is 75 years.