Factors affecting fertility and reproductive outcomes

Diet and lifestyle 20. jul 2023 4 min Written by Kristian Sjøgren

A major study shows how weight, alcohol consumption, smoking and coffee consumption affect people’s chances of getting pregnant, how many children they end up having, whether they need infertility treatment to get pregnant and the average time to conception.

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When couples want to conceive a child, various factors can prevent this from happening easily and quickly. Previous studies have shown that high body mass index (BMI) and smoking can reduce the probability of pregnancy and increase the likelihood of needing infertility treatment.

Smoking and high alcohol consumption may also indicate a risk-oriented personality, and this may be associated with an increased chance of becoming pregnant early in life. However, studies of fertility and pregnancy are often based on self-reported BMI, smoking and alcohol and coffee consumption and this can distort the results.

Researchers therefore carried out a major study examining not only what people say about their weight, smoking and alcohol and coffee consumption but also their genetic data.

The study suggests some interesting associations, which also involve a genetic predisposition for attention deficit/hyperactivity disorder (ADHD).

“One way to study how BMI, smoking, alcohol and coffee consumption affect fertility and reproductive outcomes would be to make some people smoke while others do not and then investigate their different outcomes. However, this is not ethical. In this study, we therefore tried to reduce some of the bias in self-reported smoking, BMI and alcohol and coffee consumption by using their genetic predispositions in addition to their own answers,” explains a researcher behind the study, Robyn Wootton, Lecturer, School of Psychological Science, University of Bristol, United Kingdom.

The research, published in BMC Medicine, was carried out in collaboration with Alexandra Havdahl, Norwegian Institute of Public Health and others.

Investigating fertility and reproductive outcomes using a large data bank

The researchers used the questionnaire responses and genetic data of 84,075 females and 68,002 males in Norway from the Norwegian Mother, Father and Child Cohort.

Based on the data, the researchers investigated the effect of smoking status, alcohol consumption, BMI and coffee consumption on various fertility end-points, such as time to conception, the need for infertility treatment or spontaneous abortion, or reproductive end-points, such as age at first birth and number of children.

The researchers then repeated the process using the genetic data of 1,232,091 people from the UK Biobank.

People with overweight have more difficulty getting pregnant

The results point in different directions. The researchers found that self-reported higher BMI was associated with longer time to conception, higher risk of needing infertility treatment and higher risk of spontaneous abortion.

The only correlation researchers found in the genetic data was between genetic predisposition to overweight and increased time to conception.

However, Robyn Wootton points out that, although the researchers had genetic data on many people, they could only find medium-sized effects and not small effects, and genetic predisposition for higher BMI might have a small effect on fertility-related endpoints despite the researchers not finding this.

Smoking associated with younger age at first birth

The researchers found that smoking was only weakly associated with increased time to conception.

Smoking was associated with younger age at first birth, both in the self-reported data and in the genetic data examined by the researchers on the genetic factors associated with an increased risk of smoking.

Robyn Wootton explains that other studies have also found that smoking is associated with an increased likelihood of conception at a young age and that the genetic variants thus probably capture high-risk behaviour that can lead to both smoking and unprotected sex at an early age.

“You would think that smoking makes getting pregnant difficult, but our study only found weak evidence of this, while we found that smoking was associated with younger age at first birth,” adds Robyn Wootton.

Alcohol consumption points in different directions

High alcohol consumption was associated with having a first child late and having fewer children but also with a lower risk of needing infertility treatment.

Robyn Wootton says that alcohol consumption is a difficult variable to manage in association studies, since people who do not drink alcohol often differ considerably from people who drink just a little.

Abstinence is a lifestyle choice that is often associated with other lifestyle choices that can affect both conception and the number of children.

High alcohol consumption is also often associated with low educational attainment, and longer education is associated with having fewer children and later in life.

“These associations are very common in self-reported data, and examining the genetic factors is therefore important. In our study, however, we found no association between genetic predisposition for higher alcohol consumption and fertility or reproductive outcomes,” explains Robyn Wootton.

Finally, the researchers found no association between coffee consumption and fertility or reproductive outcomes.

Contradictory results

According to Robyn Wootton, several of the results are interesting because they indicate that the data capture patterns that do not necessarily have a biological explanation.

For example, the results from both the self-reported data and the genetic data show that higher BMI results in a longer time to conception. This confirms underlying genetic and biological causality.

Conversely, higher BMI was also associated with younger age at first birth, which contradicts the fact that higher BMI both creates difficulty in getting pregnant and leads to conception at an earlier age.

“This shows that using self-reported data captures more than just biology and genetics. A social aspect affects the results, and the combination of self-reported data and genetics makes our conclusions stronger,” says Robyn Wootton.

Genes for impulsivity have an important role

Finally, the researchers also tried to find other genetic predispositions for younger age at first birth in the genetic data.

They examined known genetic markers for education and ADHD to determine whether impulsivity is associated with the likelihood of both smoking and being young at first conception.

This part of the study showed that the genetic factors associated with increased risk of ADHD are also linked to an increased likelihood of conception at a young age, just as genetic predisposition can be linked to low educational attainment.

“This suggests that when we find that smoking is associated with conception at a young age, smoking is probably not the cause but more likely socioeconomic factors and high-risk behaviour that is linked to impulsivity,” concludes Robyn Wootton.

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