Fertilization to old age

Big Picture

Fertilization creates a zygote, which undergoes cell divisions and implants in the uterus. After implantation, it becomes an embryo and forms 3 cell layers, which differentiate and develop into different organs. At nine weeks, the embryo becomes a fetus, and the fetus continues to grow and takes nutrients from the placenta. During childbirth, uterus contractions push the baby out of the mother’s body. The baby grows rapidly throughout infancy and grows more slowly during childhood. During adolescence, children develop into adults. Starting in middle adulthood, health declines as cells lose their ability to divide.

Key Terms

Fertilization: Process in which two gamete cells unite.

Gamete: Reproductive cells.

Zygote: A fertilized cell (the result of fertilization).

Blastocyst: Ball of cells with a fluid-filled cavity inside.

Implantation: Process in which a blastocyst embeds in the endometrium lining the uterus.

Embryo: Stage of growth and development that occurs from implantation through the eighth week after fertilization in humans

Differentiation: Process where an unspecialized cell becomes specialized.

Fetus: The developing human organism between week eight and birth (typically 38 weeks).

Placenta: Temporary organ connecting mother to fetus that provides fetus with necessary nutrients.

Amniotic Sac: Membrane surrounding and protecting the fetus.

Pregnancy: Carrying offspring from fertilization until birth.

Infancy: First year of life after birth.

Adolescence: Period of life between puberty and adulthood

Fertilization & Implantation

Fertilization takes place when the male gamete cell (sperm) and the female gamete cell (egg) unite

  • Sperm are deposited in the vagina during sexual intercourse
  • The sperm must reach the egg one or two days after the ovary releases the egg
  • A sperm that penetrates the egg triggers the egg to complete meiosis
  • The nucleus of the egg fuses with nucleus of sperm to create a zygote
  • While the zygote travels down the fallopian tube toward the uterus, it divides into a ball of cells called a morula
  • Fluid-filled cavity forms inside the morula–it becomes a blastocyst
  • Blastocyst implants in uterus 4-5 days after fertilization and becomes an embryo

Growth & Development: Embryo

  • Week 2: Cells in embryo form 3 layers–ectoderm, mesoderm, and endoderm
  • Week 3: Cells undergo differentiation–gene expression is regulated in cells so that certain genes are expressed while others are switched off, allowing for specialized cells to develop
  • Many factors affect gene expression and cell differentiation such as the environment and drugs taken by the mother
  • Major organs form after differentiation
  • Embryo continues to develop and grow in size - see figure on right for embryonic development during weeks 4-8
Image Credit: Melchior Meijer/,
CC-BY-SA 2.5


Fertilization to old age cont.

Growth & Development: Fetus

  • From the end of week eight until birth, the developing organism is now referred to as a fetus
  • The fetus continues to get bigger and develops organs
  • The fetus gets oxygen and nutrients from the mother and removes waste through the placenta
  • The placenta forms shortly after implantation and develops and grows along with the fetus in order to meet the needs of the growing fetus
  • The fetus is connected to the placenta through the umbilical cord
  • The amniotic sac surrounds and protects the fetus
  • The amniotic sac contains fluid that allows the fetus to move around
Image credit: Melchior Meijer/,
CC-BY-SA 2.5

Pregnancy & Childbirth

Pregnancy typically takes 38 weeks for humans.

  • The mother provides all necessary nutrients to the fetus
  • A pregnant mother must be careful about what she puts in her body; harmful substances such as drugs can cause birth defects
  • When the child is ready to be born, the amniotic sac breaks and labor begins–usually around 38 weeks after fertilization
  • Uterus walls contract, cervix dilates
  • Contractions push fetus out through vagina
  • When the baby comes out, the umbilical cord is cut and the baby no longer receives oxygen through the placenta
  • With the connection to the placenta cut, carbon dioxide builds up in the baby’s blood, which stimulates the brain to trigger breathing

From Birth to Adulthood

Infancy is the fastest period of growth after birth

  • At 6 months, babies can sit, crawl, and make sounds
  • Childhood: growth continues but begins to slow down
  • Age 6: children start developing permanent teeth, speak fluently, develop friendships

Adolescence is a period of significant changes

  • During puberty, people become physically mature
  • Adolescents become more emotionally mature and more independent

Adulthood does not have a definite starting point

  • Early adulthood ranges from 20s to early 30s
  • Middle adulthood ranges from mid-30s to mid-60s
  • People start to show physical signs of aging (wrinkles, gray hair)
  • Strength, reaction time, and vision start to decline
  • Old age ranges from mid-60s to the end of life
  • Immune system becomes less efficient and risks of serious illnesses increase

Aging occurs as cells stop dividing and die. Two reasons that cells stop dividing are:

  • Cells are programmed to divide a set number of times (they cannot divide infinitely)
  • Mutations accumulate in DNA, which hampers cell division