11.8Chapter 11: Human Genetics and Development

Smoking and pregnancy: the evidence — Alhussan Biology - Stage 9 Revision Notes & Topical Questions

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📚 Alhussan Biology - Stage 9 Hub / Topical Revision

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Lesson summary

11.8 Smoking and pregnancy: the evidence — Birth Weight and Statistical Data — Alhussan International Academy — Biology (Stages 7–9) notes: Interpret data on smoking and pregnancy and evaluate how the strength of statistical evidence supports a conclusion.Carbon monoxide — if a p…

Large studies
Stillbirth rates
Second-hand smoke
Small survey
Strength of evidence
Describing data

Section 1 of 7Effects of smoking in pregnancy

Carbon monoxide

Carbon monoxide if a pregnant woman smokes, up to 10% of her blood carries carbon monoxide, starving the fetus of oxygen.

Risks

Risks the baby may fail to grow properly, be born very small, or die before or soon after birth.

Large studies

Large studies they followed thousands of pregnant women and recorded smoking and birth weight.
On average, babies of smokers weigh about 200 g or more less than babies of non-smokers.
The more cigarettes smoked a day, the lower the average birth weight.
Smoking is also linked to more miscarriages, stillbirths and premature births, and a higher risk of cot death.

Section 2 of 7Why smoking lowers birth weight

Carbon monoxide

Carbon monoxide it binds to haemoglobin in the mother's and fetus's blood and reduces oxygen delivery.

Nicotine

Nicotine it narrows blood vessels, including those of the placenta, so less blood and fewer nutrients reach the fetus.

Result

Result the fetus grows more slowly and is born smaller.
Low birth weight babies are more likely to develop infections and breathing difficulties.

Section 3 of 7Relative risks from the data

Problem	Mother smokes	Second-hand smoke exposure
Low birth weight	200 g or more lower than non-smokers	30-45 g lower than non-smokers
Heart defects	9% more likely	Increased risk
Stillbirth	47% more likely	Possible increase
Early birth	27% more likely	Increased risk
Cleft palate	60% or more more likely	14% more likely

Stillbirth rates

Stillbirth rates they are 3.8 per 1000 births for non-smokers and 5.8 for smokers, compared with 4.2 for all mothers.

Second-hand smoke

Second-hand smoke even if the mother does not smoke, her baby can be harmed by a smoking husband or a smoky workplace.

Section 4 of 7Evaluating the evidence

Small survey

Small survey a family survey of 30 births is a small sample, so it can suggest a pattern but not prove it.
It may show more problems, such as a stillbirth or a cleft palate, among babies of mothers who smoke.

Large studies

Large studies data from hundreds of thousands of pregnant women, such as a study of 841 228 mothers, are much stronger evidence.

Strength of evidence

Strength of evidence it depends on the size of the samples and how the data are analysed.

Describing data

Describing data state the trend, quote figures with units and compare groups.

Percentage change

Percentage change divide the change by the original value, then multiply by 100.

Section 5 of 7Strengthening the conclusion

1Other factors

Other factors smokers may be younger, have poorer diets or less access to care, which could also affect birth weight.

2Statistical analysis

Statistical analysis it adjusts for such factors by comparing groups similar in age, income and health.

3Further support

Further support a known mechanism, similar results in other countries and similar birth weights when mothers stop smoking early in pregnancy support a causal link.

4Reliability

Reliability a large sample, comparison with non-smokers, accurate hospital records and repeated studies make evidence reliable,
while self-reported smoking may be under-reported.

Section 6 of 7Key terms

Term	Meaning
Stillbirth	A baby born dead.
Cleft palate	A birth defect where the roof of the mouth and lips do not form properly.
Birth weight	The mass of a baby at birth.
Relative risk	How much more or less likely an outcome is in one group than another.
Second-hand smoke	Smoke from other people's cigarettes that is breathed in.
Sample size	The number of individuals studied.

Section 7 of 7Exam technique

Saying the data prove that smoking is the only cause of low birth weight.

Forgetting to give units when quoting data.

Calculating percentage change by dividing by the new value instead of the original.

Ignoring sample size when judging reliability.

Last updated: 13 October 2026