The Beginnings of Life

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SPERM +

OVUM (released during ovulation)

= ZYGOTE

Fertilisation

Ovum’s 23 chromosomes are paired with the sperm’s 23 chromosome

Therefore, the body cells of women and men each contain 23 pairs of chromosomes

23rd pair determines sex – ovum carries X only, sperm can carry X OR Y… sex of baby depends on which chromosome the sperm carries. If X, then female, if Y, then male.

Abnormalities

Klinefelter's syndrome – XXY (1 in 1000)

Down Syndrome – 21st ‘pair’ (1 in 800)

Klinefelter’s Syndrome

Down Syndrome

Multiple births!

If two ova are released within a short time of each other and both are fertilised, two babies will be conceived and will be known as dizygotic (fraternal) twins

If a single ovum divides in two after it has been fertilised, the babies are monozygotic (identical) twins

Prenatal Development

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When?

Between conception and birth (9 months).

Most rapid and dramatic change in the entire human lifespan.

Transition from single cell functional human is influenced by factors, such as the genetic make-up of foetus and mother, and environmental factors

Three Prenatal Periods

Germinal period

Embyronic

Foetal

Germinal Period

First two weeks after fertilisation

Cell divides, becomes more complex and attaches itself to the wall of the uterus.

Period of rapid growth

72 hours after fertilisation, the zygote has divided into 32 cells, and one day later, it has 70 cells.

By the end of the second week, the amniotic sac (which protects the baby), the umbilical cord (connects baby to the placenta) and the placenta itself (which attaches the baby to the wall of the uterus and provides nutrients, oxygen and waste products to be exchanged) are developed and the cell mass has become an embryo.

Embryonic Period

•From start of third week-approximately eighth to twelfth week

•Development of major systems such as the respiratory, digestive and nervous system.

•Embryo is particularly vulnerable to harmful environmental influences (drugs, inadequate nourishment, maternal disease)

•Critical stage – almost all developmental birth defects occur during the first three months after conception

•By the eighth week, it has a prominent head, distinctive facial features, and is now called a foetus

Foetal Stage Lasts between the eighth and twelfth week until birth.

Bone and muscle tissue form and various organs and body systems continue to mature.

By third month, baby starts moving arms, kicking legs, making fists, twisting its body, turning somersaults and sucking its thumbs.

By fourth month, external body parts (e.g. fingernails, eyelashes, eyebrows) form and mother can feel movements.

By 30 weeks, the baby’s survival outside the womb may be possible with exceptional medical care.

In the last month, lungs are able to extract oxygen from air, eyes are responsive to light, electrical activity in the brain indicates sleep and wakefulness, reflexes become highly responsive

Normal birth occurs at week 38/39