Compared with having no children, the risk reduction for endometrial cancer was 21% with having one child, 38% with having two, and 51% with having three, Gunn-Helen Moen, MSc, PhD, a research fellow at the University of Queensland Institute for Molecular Bioscience in St. Lucia, Australia, and the senior author of the study, said in an email.
Dr. Gunn-Helen Moen
In the United States, the prevalence of endometrial cancer is 25.7 per 100,000 women per year, with a lifetime risk of 2.8%.
Multiple observational studies have linked giving birth to risk of endometrial cancer. For the new study, Dr. Moen and her team assessed various risk factors related to ovulation and reproductive function using Mendelian randomization, an epidemiological technique that deploys genetic variants to detect cause-and-effect relationships between potentially modifiable risk factors and health outcomes in observational data.
The researcher published their findings in BMC Medicine.
Leverage genetic data
The study used detailed genetic and health data from the UK Biobank, a databank with more than half a million participants. Genetic variants related to some of the risk factors were used to assess whether the variants make people more likely to develop endometrial cancer.
Genomewide significant single-nucleotide polymorphisms (SNPs) related to number of live births, age at menopause and menarche, and body mass index (BMI) had been identified in previous studies, the researchers reported. They conducted genomewide association analyses of the databank to identify SNPs associated with years ovulating, years using the contraceptive pill, and age at last live birth.
The MR analysis showed a potential causal effect for the number of live births (inverse variance–weighted odds ratio, 0.537) and number of years ovulating (IVW OR, 1.051), in addition to known risk factors of BMI, age at menarche, and age at menopause.
A further multivariable MR analysis showed that number of births had a negative causal effect on endometrial cancer risk (OR, 0.783), independent of the causal effect of known risk factors such as BMI, age at menarche and age at menopause.
Reported limitations included being unable to perform MR analyses on some factors, such as oral contraceptive use, because of a lack of valid genetic instruments. The researchers could not perform an age adjustment at diagnosis because of a lack of data.
In addition, the genetic data came exclusively from White women of European ancestry.
‘A personal choice’
Other investigators have hypothesized that the protective effect of childbirth may be caused by shedding of malignant and premalignant endometrial cells during and after childbirth and exposure to high levels of progesterone in late stages of pregnancy, the research team noted.
Dr. Moen said, based on the results, physicians might consider number of childbirths in assessing a patient’s risk of endometrial cancer.
However, Britton Trabert, MSPH, MS, PhD, an epidemiologist and assistant professor of obstetrics and gynecology at the University of Utah, Salt Lake City, said it’s unlikely the findings will affect clinical practice given that they “largely replicate well-characterized endometrial cancer risk associations.”
“Pregnancy and childbirth are a personal choice and is not largely regarded as a modifiable factor for cancer prevention,” said Dr. Trabert, who was not involved in the study.
The study’s investigators reported funding from the governments of Australia, Norway and the United Kingdom and the British Heart Foundation. No financial conflicts of interest were reported. Dr. Trabert reported no relevant financial interests.