Pregnancy, the Placenta and Fetal Development
Pregnancy, the placenta and fetal development is IGCSE-specific detail on what happens after fertilisation: implantation of the embryo, the structure and function of the placenta, amnion and umbilical cord, and the exchange of substances between mother and fetus across the placenta by diffusion and active transport. Beyond GCSE, students must explain why the placenta membrane must be thin with a large surface area, name what crosses it in each direction (oxygen, glucose, amino acids and antibodies to the fetus; carbon dioxide and urea from the fetus), and explain why harmful substances such as alcohol, nicotine and some pathogens can also cross and damage fetal development.
Before you start
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Method
- Start the sequence at implantation: after fertilisation, the zygote divides repeatedly by mitosis as it travels down the oviduct to the uterus, forming a ball of cells (embryo) that embeds itself in the thickened, blood-rich uterus lining (endometrium); this is implantation.
- Name and give the function of each supporting structure: the placenta (a disc-shaped organ formed from both maternal and fetal tissue, where substances are exchanged between the mother's blood and the fetus's blood); the umbilical cord (contains blood vessels connecting the fetus to the placenta); the amnion (a membrane that surrounds the fetus, enclosing the amniotic fluid); amniotic fluid (a watery fluid that cushions/protects the fetus from physical shocks and allows it to move).
- Explain the placenta's structure in terms of exchange efficiency: it has a very large surface area and a thin membrane separating maternal and fetal blood, and is well supplied with blood vessels, which together maximise the rate of diffusion of substances between the two blood supplies, while the mother's and fetus's blood do not normally mix.
- List what moves in each direction and by what process: from mother to fetus, oxygen, glucose, amino acids, and antibodies (providing some passive immunity) diffuse or are actively transported across the placenta; from fetus to mother, carbon dioxide and urea (waste from the fetus's metabolism) diffuse across the placenta to be removed by the mother's excretory system.
- For questions on why harmful substances affect the fetus, explain that the placenta does not filter out every substance in the mother's blood: alcohol and nicotine are small enough to cross the placenta and can restrict fetal growth or affect development (for example, nicotine constricts blood vessels, reducing oxygen supply to the fetus), and some pathogens (such as rubella virus) can also cross and cause birth defects if the mother is infected during pregnancy.
- When a question gives a graph or table of birth mass against a maternal variable (for example, cigarettes smoked per day), describe the trend using the data (state the direction and use figures from the table/graph) before explaining the biological mechanism linking the variable to birth mass.
Worked example
A study compared the average birth mass of babies born to mothers who smoked during pregnancy with babies born to non-smoking mothers. Non-smoking mothers' babies had an average birth mass of 3.4 kg; smoking mothers' babies had an average birth mass of 2.9 kg. Calculate the percentage decrease in average birth mass associated with smoking, and explain a biological mechanism that could cause this difference.
- Write down the percentage change formula: percentage change = (new value - original value) / original value x 100.
- Substitute the values, taking the non-smoking group as the original value: (2.9 - 3.4) / 3.4 x 100.
- Calculate: -0.5 / 3.4 x 100 = -14.7 (to 3 s.f.).
- State the final calculated answer: a 14.7% decrease in average birth mass.
- Explain a mechanism: nicotine from cigarette smoke crosses the placenta and constricts (narrows) blood vessels, reducing the blood supply and therefore the oxygen and nutrient supply reaching the fetus across the placenta.
- Link the mechanism to the outcome: reduced oxygen and nutrient supply restricts the growth of the fetus, which can explain the lower average birth mass in the smoking mothers' babies.
Practice questions
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Q1Name the process by which the embryo embeds itself in the uterus lining.Show answer
Answer: Implantation.
Q2Name the structure that connects the fetus to the placenta.Show answer
Answer: The umbilical cord.
Q3State the function of amniotic fluid.Show answer
Answer: It cushions and protects the fetus from physical shocks/knocks, and allows it to move.
Q4State two substances that diffuse from the mother's blood to the fetus's blood across the placenta.Show answer
Answer: Any two of: oxygen, glucose, amino acids, antibodies.
Q5State one substance that passes from the fetus's blood to the mother's blood across the placenta, and explain why it needs to be removed.Show answer
Answer: Carbon dioxide (or urea); it is a waste product of the fetus's metabolism/respiration and would be toxic/harmful if it built up in the fetus's blood.
Q6Explain why the mother's blood and the fetus's blood do not normally mix, even though substances pass between them.Show answer
Answer: The two blood supplies are separated by the thin membrane of the placenta; substances diffuse (or are actively transported) across this membrane rather than the blood itself crossing over, which also protects the fetus from potentially harmful cells or antibodies that could attack it as foreign tissue.
Exam-style questions
Written in the style of a IGCSE Science exam paper, with a full mark scheme.
State two features of the placenta that allow efficient exchange of substances between mother and fetus.
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A midwife records the head circumference of fetuses at 30 weeks of pregnancy in two groups of mothers: those who tested positive for rubella infection in the first trimester (average head circumference 26.1 cm) and those who tested negative (average head circumference 28.4 cm). Calculate the percentage difference between the two groups, and explain how a maternal infection could cause this difference.
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Explain how the structure of the placenta enables the exchange of substances between mother and fetus, and explain why substances such as alcohol and pathogens can also cross the placenta and affect fetal development.
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Free printable worksheet
Want more practice on paper? Download the pregnancy, the placenta and fetal development worksheet pack - 5 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 4 of IGCSE Science Workbook, the whole course as one free printable PDF.
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