Infertility in men is linked to their weight at birth, study shows babies below 6.6 lbs are at an increased risk
Among the men who were born small, researchers found that 8.3% had been diagnosed or were being treated for infertility as compared to 5.7% of men born with the appropriate weight
Boys who are born weighing less than 6.6 lbs are 55% more likely to be infertile in adulthood as compared to boys born at an average weight, according to a new study.
The author of the study, Anne Thorsted, told MEA WorldWide (MEAWW) that sometimes one must look at the very early life to find explanations of health problems that occur later in life.
“In our study, we refer to babies who are smaller than usual for the number of weeks of gestation. Birth weight is caused by other factors (for example maternal factors during pregnancy) and if we want to intervene against 'infertility', then these other factors may be explored further. However, this presupposes a causal relationship,” Thorsted, who conducted the study when she was part of a research group from the Department of Public Health at Aarhus University, Denmark, said.
Gestational age is the common term used during pregnancy to describe how far along the pregnancy is. It is measured in weeks: a normal pregnancy can range from 38 to 42 weeks.
According to the research team, an estimated 12.5% of all couples are affected by infertility, defined by unsuccessfully attempting pregnancy for a year or longer.
Out of 100 couples in the US, 12 to 13 have trouble conceiving, says the US Department of Health & Human Services. About one-third of infertility cases are caused by fertility problems in men and another one-third of fertility problems are due to fertility problems in women.
The researchers say that there are numerous causes of infertility, but in one-third of the cases, a specific cause cannot be identified despite thorough clinical examination.
The researchers obtained information on birth weight and gestational age from birth records. They retrieved information on infertility diagnoses and fertility treatments from the Danish National Patient Registry (NPR), established in 1977, and the Danish In Vitro Fertilisation registry.
Information on infertility was obtained during the period from the participants’ 18th birthday and up until December 31, 2017.
The study population included children, born of mothers part of the ‘Healthy Habits for Two’ cohort, established from April 1984 to April 1987 in two cities (Aalborg and Odense) in Denmark. A total of 11,980 pregnant women participated. The final study consisted of 10,936 individuals -- 5,594 men and 5,342 women born between 1984 and 1987 -- followed until December 31, 2017.
The mothers of all the participants had completed a questionnaire during pregnancy that asked about factors that could affect the results, such as age, lifestyle (smoking and alcohol consumption), health (including body mass index) and sociodemographic status (employment and whether or not the parents lived together). The average age of the participants was 32 years at the end of follow-up.
The researchers found that men who were born underweight at full-term or small for gestational age had a 55% increased risk of being diagnosed with or treated for infertility compared to men born appropriate for gestational age. No association was found between women’s birth weight for gestational age and risk of infertility, says the study published in Human Reproduction.
“Among the men who were born small for gestational age, we found that 8.3% had been diagnosed or were being treated for infertility by the end of 2017 compared to 5.7% of men born with the appropriate weight," says Thorsted.
According to the researchers, small for gestational age is defined as babies having birth weights in the bottom 10% according to each week of gestation compared to other children of the same gestational age. For instance, the weight range for babies born at full-term (40 weeks) was between approximately 2500-4500 grams (5.5-9.9 Ibs) in this study. Hence, full-term babies born weighing less than 3000 grams (6.6 lbs) were defined as being small for gestational age and weighing less than 90% of other children of the same gestational age.
Previous research has suggested a link between restricted growth of the fetus in the womb and a two to three times increased risk of penis and testicular problems in boys; in particular, hypospadias (a malformation where the opening of the urethra is not at the tip of the penis), cryptorchidism (undescended testis) and testicular cancer. These conditions are linked to fertility problems. However, says the research team, there have been few and conflicting studies on the link between birth weight for gestational age and infertility.
In the current study, when the researchers excluded men born with hypospadias or cryptorchidism, the link between being born small for gestational age and infertility weakened.
“In one of our sub-analyses, we excluded men born with the genital malformations; hypospadias and cryptorchidism. In this analysis, the risk of infertility among men born small for gestational age attenuated, indicating that the genital malformations may account for a part of the observed association. It could also be speculated that cryptorchidism, hypospadias and reduced infertility have common origins in fetal life,” Thorsted told MEAWW.
The research team says it was not yet clear what the potential mechanisms could be for the link between birth weight and infertility. They say “genital malformations may account for part of the observed association,” but this must be explored further.
"A suboptimal growth environment for the fetus, for whatever reason, could itself be detrimental to the development of sperm production and reproductive organs," says Thorsted.
She says: "It could also be speculated that the mother's health and lifestyle during pregnancy could affect both fetal growth and the development of reproductive functions. For instance, we already know that if the mother smokes, this can have an impact on the fetus. It may well be that cryptorchidism, hypospadias, and infertility have common origins in fetal life.”
The research team says that since the participants had not reached the end of their reproductive life by 2017, it would be 'interesting' to see what the situation was in another 10 years' time.