In a recent study published within the journal BMC Medicine, researchers evaluate the effect of lifestyle behaviors on various fertility and reproductive outcomes.

Study: Associations between health behaviours, fertility and reproductive outcomes: triangulation of evidence within the Norwegian Mother, Father and Child Cohort Study (MoBa). Image Credit: Chinnapong / Shutterstock.com

Aspects contributing to infertility

Fertility guidelines include smoking cessation, limiting the consumption of caffeine and alcohol, and weight management. Heavy smoking and increased alcohol intake reduce the likelihood of conception, whereas obesity and increased caffeine intake increase miscarriage risks.

In males, alcohol intake and smoking lower semen quality, whereas obesity increases the time to conception and lowers the probability of natural conception. Nevertheless, fertility-related advice is basically based on observational studies, often analyzing data with confounding bias from weight loss plan, sleep, and physical exertion.

Concerning the study

In the current study, researchers explore the relationships between lifestyle decisions and reproductive and fertility outcomes amongst men and ladies using a Mendelian randomization (MR) approach to account for potential confounding and reverse causation.

The study utilized data from 84,705 Norwegian mother, father, and child (MoBa) study participants, 68,002 of whom were female.

Multivariate regression modeling of lifestyle decisions, including caffeine and alcohol intake, smoking, and body mass index (BMI), was performed to evaluate the self-reported study outcomes. The assessed fertility outcomes included infertility therapy usage, miscarriage, and the time for conception, whereas the reproductive outcomes were age at first delivery and number of youngsters.

Data were adjusted for the 12 months of birth, attention-deficit hyperactivity disorder (ADHD) traits, and level of education. Individual-level MR was used to explore probable causal effects of lifestyle decisions on reproductive/fertility outcomes amongst 63,376 women and 45,460 men.

As well as, summary-level multivariate MR was performed for available study outcomes amongst the UK Biobank participants, controlling for educational level and ADHD liability.

The study utilized measures from the primary questionnaires received between weeks 13 and 17 of gestation to acquire data on previous deliveries, medical history, occupation, medications, home and workplace exposures, mental well-being, and lifestyle decisions.

Questionnaire data were linked to the Medical Birth Registry of Norway (MBRN). Men and women documented their health behavior three and 6 months before pregnancy, respectively.

Blood samples were obtained from MoBa study parents while pregnant. Probably the most recently released quality-controlled genotype data for MoBa study participants were used.

The MR approach included multivariate regression evaluation, followed by individual-level MR and summary-level Mendelian randomization. For individual-level MR analyses, genetic scores were constructed using genome-wide association studies (GWAS) summary statistics, whereas, for summary-level Mendelian randomization, individual single nucleotide variant (SNP) effect size data were utilized.

Results

Multivariate regression modeling showed that elevated BMI values increased the time to conception, the necessity for infertility therapy, and the probabilities of miscarriage. As well as, the time to conception was higher amongst smokers. Robust evidence was obtained for the results of smoking and elevated BMI on lower age at first delivery, in addition to elevated BMI on longer time for conception.

Weak evidence was obtained for the results of smoking on longer times to conception. Similar associations were obtained for age throughout the first delivery within the summary-level Mendelian randomization analyses; nevertheless, attenuated effects were observed within the multivariate MR analyses. The study outcomes didn’t show significant differences between men and ladies, apart from parent age at first birth, which was greater amongst men.

Greater age at first delivery was related to having fewer children, longer times to conception, in addition to increased probabilities of miscarriage and infertility therapy requirements. Having more children was related to shorter times to conception, a greater likelihood of miscarriage, and using infertility therapy.

Greater frequency of alcohol intake and binge drinking were each related to having fewer children and greater age at first delivery. Individuals with greater alcohol intake were less prone to require infertility treatment, with odds ratios (OR) for alcohol frequency and binge drinking of 0.9 and 0.8, respectively. Greater caffeine intake was related to lower age at first birth amongst men and a lower likelihood of requiring treatment amongst women (OR 1.0).

Cigarette use was related to having fewer children and lower age at first delivery. Each cigarette smoking measures were related to longer times to conception for girls.

Greater BMI values were related to having fewer children. Elevated BMI was related to greater age at first birth amongst men and lower age at first birth amongst women.

Higher BMI was related to longer times to conceive, greater likelihood of requiring infertility therapy (OR 1.0), and having miscarriages (OR 1.0). Within the individual-level MR analyses, genetically estimated elevated BMI values were related to lower age at first birth, as were genetic liabilities for the initiation of cigarette smoking initiation.

There was weak evidence for an association between genetically estimated greater alcohol intake and lower age at first birth amongst men. Genetically estimated elevated BMI was related to longer times to conception amongst women.

Similarly, weak evidence was presented for genetic liability to smoking initiation and genetically estimated greater alcohol intake, each related to longer times to conceive. Within the summary-level Mendelian randomization analyses, strong evidence was obtained for the results of smoking initiation on lower age at first birth for girls, a greater number of youngsters, and fewer miscarriages.

Conclusions

The study findings showed essentially the most consistent associations between smoking habits and BMI values and increased time to conception and lower age throughout the first delivery. The positive association between the age at first delivery and time to conception indicated that the mechanisms for reproductive outcomes and fertility outcomes were distinct.

Multivariate MR evaluation findings indicated that the results of age at first delivery could be explained by underlying educational and ADHD liabilities. Taken together, the study findings could inform fertility guidance.

Journal reference:

• Wootton, R. E., Lawn, R. B., Magnus, M. C. et al. (2023). Associations between health behaviours, fertility and reproductive outcomes: triangulation of evidence within the Norwegian Mother, Father and Child Cohort Study (MoBa). BMC Medicine 21(125). doi:10.1186/s12916-023-02831-9