Low male birth weight indicates lower fertility

Weight at birth seems to be something of an indicator of future fertility for men, but not for women. A Danish study has found that boys weighing less than 3Kg at birth, could suffer infertility later in life. The study found that by their early 30s, 8.3% of men who had been small babies had fertility issues. This compared to only 5.7% of those who were born in the normal weight spectrum.

How is this so?

The investigators speculate that poor growth in the womb can lead to poor development of the testes. This in turn can lead to problems with sperm production. The study of followed 5,500 men and 5,300 women born between 1984-1987 until 2017. They found that infertility in women was not linked to weight at birth. About 10% of men and women were born small for their age. 80% were a normal weight at birth and nearly 9% were born larger.

Smoking link

Anne Thorsted, Lead Researcher, Aarhus University, said it was unclear what lay behind the link between birth weight and infertility. However, mothers of low-weight babies had the highest level of cigarette and alcohol consumption. Smoking is known to cause lower birth weights.

Two specific conditions were found to inhibit fertility in the men who were small babies later in life:

Hypospadias: a deformity where the opening of the urethra is not at the tip of the penis
Cryptorchidism (un-descended testis) were behind most of the male fertility problems.

Ms. Thorsted, speculating on the causes said, “A sub-optimal growth environment for the foetus, for whatever reason, could itself be detrimental. Especially for the development of sperm production and reproductive organs. The mother’s health and lifestyle during pregnancy could affect both foetal growth and the development of reproductive functions. For instance, we know already that if the mother smokes, this can have an impact on the foetus. Our results show that we can find explanations for later life health problems by looking back to the very early life.”

Critical parts of a man’s fertility actually occurs before he was ever born

Professor Allan Pacey, Professor of Andrology, University of Sheffield agreed. He said, “It’s important to note that this observation does not demonstrate cause and effect. Although the theory that the most critical parts of a man’s fertility actually occurs before he was ever born fits. This is a bit hard to get one’s head around as a concept. But there is a lot of evidence emerging to support the idea. This new study now forms an important part of that theory.”

The researchers also acknowledge that the study had some limitations. For instance, the average age of people in the study was 32. This means that they had not yet reached the end of their reproductive years. A follow-up study in 10 years time may change the conclusions.