Reproduction, Pregnancy & Fetal Development
Section: Biology | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
Sexual Reproduction
- Sexual reproduction is the process where male and female gametes fuse to create offspring
- this produces a fertilised egg with a new combination of DNA from both parents, creating genetic variation
Gametes (Sex Cells)
- Gametes are specialised sex cells that carry genetic information from each parent
| Feature | Male Gamete | Female Gamete |
|---|---|---|
| Name | Sperm cell | Egg cell (ovum) |
| Produced in | Testes | Ovaries |
| Size | Small, mobile | Large, stationary |
| Number of chromosomes | 23 (haploid) | 23 (haploid) |
| Special features | Tail for swimming, many mitochondria for energy | Large cytoplasm with food stores |
Sperm cell and egg cell compared, showing their relative size and structure
Fertilization
- Fertilisation is the fusion of a sperm nucleus with an egg nucleus
- this creates a fertilised egg (zygote) with a complete set of 46 chromosomes
The Fertilization Process
- Sperm swims to the egg in the oviduct (fallopian tube)
- Sperm head penetrates the egg membrane
- Sperm nucleus fuses with egg nucleus
- this forms a fertilised egg (zygote) with 46 chromosomes, 23 from each parent
- the new combination of DNA creates unique offspring
- Sperm (23 chromosomes) + egg (23 chromosomes) = fertilised egg (46 chromosomes)
The stages of fertilisation, from sperm penetration to the formation of a 46-chromosome zygote
Pregnancy and Fetal Development
- Pregnancy is the period from fertilisation to birth, lasting approximately 40 weeks
- the developing baby progresses through distinct stages as organs form and mature
Development Timeline
| Time | Stage | What Happens |
|---|---|---|
| 0-5 days | Fertilised egg → embryo | Cell divides repeatedly; a ball of cells forms |
| 6-7 days | Implantation | Embryo embeds in the uterus lining |
| Week 2-8 | Embryo | Organs begin to form; heart starts beating |
| Week 9-40 | Fetus | Growth and development; organs mature |
| Week 40 | Birth | Baby is born |
The Placenta and Umbilical Cord
- The placenta is an organ that connects mother and fetus, allowing exchange of substances
- The umbilical cord connects the fetus to the placenta
| Function | How It Works |
|---|---|
| Nutrient supply | Glucose, amino acids, minerals pass from mother's blood → fetus |
| Gas exchange | Oxygen from mother → fetus; CO₂ from fetus → mother |
| Waste removal | Urea from fetus → mother's blood for excretion |
| Protection | Antibodies pass from mother → fetus for immunity |
| Barrier | Prevents mother's and fetus's blood from mixing (harmful substances blocked) |
- Mother's blood and fetus's blood do not mix
- substances pass across the placenta by diffusion
How the placenta exchanges substances between mother and fetus without their blood mixing
Factors Affecting Fetal Development
- The health of the mother directly affects fetal development during pregnancy
- several factors can have positive or negative impacts on the developing baby
Diet and Nutrition
- Proper nutrition during pregnancy is essential for healthy fetal development
| Nutrient | Why Needed | Deficiency Effect on Fetus |
|---|---|---|
| Protein | Growth and cell formation | Poor growth, low birth weight |
| Calcium | Bone and tooth development | Weak bones, poor skeletal development |
| Iron | Making red blood cells | Anaemia, low oxygen supply to fetus |
| Folic acid (Vitamin B9) | Neural tube development (brain and spine) | Birth defects (spina bifida) |
| Vitamin D | Calcium absorption | Weak bones, rickets |
- Malnutrition during pregnancy can cause:
- low birth weight
- premature birth
- increased risk of infant mortality
- developmental delays
Smoking During Pregnancy
- Smoking during pregnancy exposes the fetus to harmful chemicals that reduce oxygen and nutrient supply
- tobacco contains nicotine, carbon monoxide, and tar, all of which cross the placenta
- Nicotine: constricts blood vessels, reducing oxygen and nutrient supply
- Carbon monoxide: reduces the oxygen-carrying capacity of the blood
- Tar and other chemicals: cross the placenta and harm fetal development
- Effects on the fetus:
- low birth weight
- premature birth
- increased risk of stillbirth
- respiratory problems
- increased risk of sudden infant death syndrome (SIDS)
- developmental issues
Alcohol During Pregnancy
- Alcohol crosses the placenta and directly affects the developing fetus
- this can cause fetal alcohol syndrome, with permanent effects
- Effects of fetal alcohol syndrome:
- brain damage
- learning difficulties
- physical abnormalities — facial features, growth problems
- behavioural problems
- low birth weight
- There is no safe level of alcohol consumption during pregnancy
- pregnant women should avoid alcohol completely
Drugs During Pregnancy
- Many drugs can cross the placenta and harm fetal development
| Drug Type | Examples | Effects on Fetus |
|---|---|---|
| Illegal drugs | Cocaine, heroin, cannabis | Birth defects, premature birth, low birth weight, withdrawal symptoms, developmental problems |
| Some medications | Certain antibiotics, pain relievers | Birth defects if taken without medical advice |
| Caffeine (excess) | Coffee, energy drinks | High doses: low birth weight, premature birth |
Stress and Infections/Disease
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