Life Sciences: Human Reproduction- Gestation (Pregnancy)

Hi

Gestation is the time between conception (when sperm meets egg) and birth- during this the fetus is developing in the uterus.

• Early development (0-end of 2 months)
  • Fertilisation
  • Blastocyst and implantation formation
  • Embryo formation
• Later development (3-9 months)
  • Fetal stage

EARLY DEVELOPMENT
FERTILSATION

1. Occurs in the fallopian tube
2. Nuclei of sperm and egg fuse to form a diploid zygote
3. Sperm move through cervix to the uterus and then fallopian tubes
4. Fertilisation (egg) membrane forms immediately to stop other sperm cells from entering the egg

BLASTOCYST FORMATION & IMPLANTATION

1. Diploid zygote divides immediately to form 2 cells (mitosis)
2. Each of these 2 cells divides repeatedly to form a solid ball of cells (morula)
3. The morula develops into a hollow, fluid-filled ball of cells called a blastocyst
4. After a few days, the blastocyst sinks into the endometrium and this is called implantation (about 10 days after ovulation)

FORMATION OF THE EMBRYO

1. Lasts from 3rd week- the 8th week after conception
2. Inner cell mass forms the embryo, amnion & yolk sac
3. Trophoblast forms the placenta through development of endometrium
4. Embryo develops into 3 layers- ectoderm, mesoderm & endotherm

WHAT IS THE DIFFERENCE BETWEEN AN EMBRYO AND A FETUS?

Embryo refers to the developing zygote until 8 weeks and the fetus is from 9 weeks where the organs have already formed.

THE PLACENTA
Connects the blood supply between the mother and the baby – they do not mix however.

1. Oxygen & dissolved foods pass from mother to child
2. Fetal metabolic waste is passed into the mother’s bloodstream
3. Maternal antibodies are passed on
4. Acts as a barrier
5. Has an endocrine function- secretes progesterone and oestrogen from the ovaries (corpus luteum to be exact) and these maintain the pregnancy. Later in the pregnancy- relaxin is released and this helps deliver the baby by relaxing the joints and ligaments

AMNION

1. Thin but strong membrane
2. Secretes amniotic fluid

AMNIOTIC FLUID

1. Supports fetus
2. Cushions and protects
3. Provides a medium to practice breathing and swallowing
4. Holds urine
5. Protects against temperature changes and dehydration

UMBILICAL CORD

1. Umbilical artery takes metabolic waste away
2. Umbilical vein brings food and other things from the mother

BIRTH

1. Dilation of the cervix- 10 cm is normally fully dilated, increased pressure causes the amnion to break and amniotic fluid is expelled (water broke)
2. Delivery of the baby
3. Expulsion of the placenta (after birth)