Human Reproduction: Conception

Conception

• Ovulation occurs halfway through the woman's menstrual cycle, aided by the release of hormones.
• The hormones also cause the lining of the uterus to grow thicker and more suitable for implantation of a fertilized egg.
• If the woman has had sexual intercourse within one or two days of the egg's maturation, one of the up to 500 million sperm deposited by the man's ejaculation may fertilize the egg.

Prenatal Development

• The three stages of prenatal development are:
• Zygote (conception to 2 weeks)
• Embryo (2 to 8 weeks)
• Fetus (8 weeks to birth)

The Zygote

• Within several hours of conception, half of the 23 chromosomes from the egg and half of the 23 chromosomes from the sperm fuse together, creating a zygote.
• The zygote travels down the fallopian tube to the uterus.
• If the zygote is viable, it will attach itself to the wall of the uterus, but if it's not, it will be flushed out in the woman's menstrual flow.
• The cells in the zygote continue to divide and differentiate, eventually forming the developing human being and the protective environment.

The Embryo

• Once the zygote attaches to the wall of the uterus, it is known as the embryo.
• During the embryonic phase, the major internal and external organs are formed, each beginning at the microscopic level.
• The changes in the embryo's appearance continue rapidly from this point until birth.
• The outer layer of embryonic cells forms the surrounding protective environment, consisting of:
• The amniotic sac: a fluid-filled reservoir that acts as a cushion and temperature regulator.
• The placenta: an organ that allows the exchange of nutrients between the embryo and the mother, while filtering out harmful material.
• The umbilical cord: links the embryo directly to the placenta and transfers all material to the fetus.

The Fetus

• Beginning in the ninth week after conception, the embryo becomes a fetus.
• The defining characteristic of the fetal stage is growth.
• The fetus grows from weighing less than an ounce to weighing an average of six to eight pounds.
• The fetus begins to take on many characteristics of a human being, including:
• Moving (by the third month)
• Sleeping
• Early forms of swallowing and breathing
• Developing senses (taste, sound, etc.)
• Research has found that the fetus develops initial preferences, such as:
• Preferring the mother's voice to that of a stranger
• Preferring the languages heard in the womb over other languages
• Preferring the kinds of foods that the mother ate during the pregnancy

Month-by-Month Development

• See the chart for detailed month-by-month changes in the development of the fetus.

Factors Affecting the Growing Fetus

• Environmental factors affect the development of the fetus during each prenatal stage.
• The developing fetus is completely dependent on the mother for life, and it is important that the mother receives prenatal care.
• Factors that can affect the growing fetus include:
• Environmental Risks:
• Teratogens (e.g., maternal diseases, drugs, alcohol, stress)
• Environmental and occupational exposures
• Environmental Chemicals:
• Pollution
• Organic mercury compounds
• Herbicides
• Industrial solvents
• Sexually Transmitted Infections:
• Complicate pregnancy and may have serious effects on both the mother and developing baby
• Maternal Diseases:
• Increase the chance that a baby will be born with a birth defect or have a chronic health problem
• Maternal Stress:
• Can leave lasting effects on a developing fetus
• Genetics:
• Genetic make-up plays a role in the impact a particular teratogen might have on a child
• Genetic hazards to development include inheritance of an extra chromosome (e.g., Down's syndrome), a defective gene on the X chromosome (e.g., hemophilia), and certain recessive genes (e.g., sickle-cell anemia)

Conception

• Ovulation occurs halfway through the woman's menstrual cycle, aided by the release of hormones.
• The hormones also cause the lining of the uterus to grow thicker and more suitable for implantation of a fertilized egg.
• If the woman has had sexual intercourse within one or two days of the egg's maturation, one of the up to 500 million sperm deposited by the man's ejaculation may fertilize the egg.

Prenatal Development

• The three stages of prenatal development are:
• Zygote (conception to 2 weeks)
• Embryo (2 to 8 weeks)
• Fetus (8 weeks to birth)

The Zygote

• Within several hours of conception, half of the 23 chromosomes from the egg and half of the 23 chromosomes from the sperm fuse together, creating a zygote.
• The zygote travels down the fallopian tube to the uterus.
• If the zygote is viable, it will attach itself to the wall of the uterus, but if it's not, it will be flushed out in the woman's menstrual flow.
• The cells in the zygote continue to divide and differentiate, eventually forming the developing human being and the protective environment.

The Embryo

• Once the zygote attaches to the wall of the uterus, it is known as the embryo.
• During the embryonic phase, the major internal and external organs are formed, each beginning at the microscopic level.
• The changes in the embryo's appearance continue rapidly from this point until birth.
• The outer layer of embryonic cells forms the surrounding protective environment, consisting of:
• The amniotic sac: a fluid-filled reservoir that acts as a cushion and temperature regulator.
• The placenta: an organ that allows the exchange of nutrients between the embryo and the mother, while filtering out harmful material.
• The umbilical cord: links the embryo directly to the placenta and transfers all material to the fetus.

The Fetus

• Beginning in the ninth week after conception, the embryo becomes a fetus.
• The defining characteristic of the fetal stage is growth.
• The fetus grows from weighing less than an ounce to weighing an average of six to eight pounds.
• The fetus begins to take on many characteristics of a human being, including:
• Moving (by the third month)
• Sleeping
• Early forms of swallowing and breathing
• Developing senses (taste, sound, etc.)
• Research has found that the fetus develops initial preferences, such as:
• Preferring the mother's voice to that of a stranger
• Preferring the languages heard in the womb over other languages
• Preferring the kinds of foods that the mother ate during the pregnancy

Month-by-Month Development

• See the chart for detailed month-by-month changes in the development of the fetus.

Factors Affecting the Growing Fetus

• Environmental factors affect the development of the fetus during each prenatal stage.
• The developing fetus is completely dependent on the mother for life, and it is important that the mother receives prenatal care.
• Factors that can affect the growing fetus include:
• Environmental Risks:
• Teratogens (e.g., maternal diseases, drugs, alcohol, stress)
• Environmental and occupational exposures
• Environmental Chemicals:
• Pollution
• Organic mercury compounds
• Herbicides
• Industrial solvents
• Sexually Transmitted Infections:
• Complicate pregnancy and may have serious effects on both the mother and developing baby
• Maternal Diseases:
• Increase the chance that a baby will be born with a birth defect or have a chronic health problem
• Maternal Stress:
• Can leave lasting effects on a developing fetus
• Genetics:
• Genetic make-up plays a role in the impact a particular teratogen might have on a child
• Genetic hazards to development include inheritance of an extra chromosome (e.g., Down's syndrome), a defective gene on the X chromosome (e.g., hemophilia), and certain recessive genes (e.g., sickle-cell anemia)

Learn about the process of conception in humans, including ovulation, hormonal changes, and uterine lining preparation. Quiz yourself on the fascinating biology of human reproduction!