Effect of father’s age on offspring

Men over 60 “have healthier babies than teen fathers” reads the headline in the Daily Mail . The newspaper reported that a study has shown that older men have healthier babies than their “teenage counterparts”. The newspaper goes on to say that even for men in their 60s, the health risks for the child are no higher than average, and in contrast, babies with fathers in their teens have more chance of being premature or dying before their first birthday.

BBC News also covered the story and said that the babies of teenage fathers had an increased chance of a low birth weight, being small for their age, and being born prematurely.

Analysis of the study reveals that older fathers were not actually compared directly with their teenage counterparts, but instead, all groups, young and old, were compared to an age group 20 to 29 years of age. This age group was used as a reference point as it has been shown to have the lowest risk of adverse birth outcomes. This was also the age group of all the mothers and naturally contained most of the pregnancies and all those where mother and father were of the similar ages. When the comparison between teenage fathers and the reference group was made, the researchers found a small difference in birth outcomes.

Although the researchers say there may be a biological reason behind the differences, such as less numerous and healthy sperm in the younger age group, they also list other possible socioeconomic and lifestyle reasons. These include the higher likelihood that younger fathers come from economically disadvantaged families and have lower educational attainment. Other aspects of the social environment, such as domestic violence, drug and alcohol abuse and smoking and passive smoking (smoking in the same room as a baby) also contribute to a higher risk of adverse birth outcomes. It seems more likely that it is these factors, rather than the father's age, that account for the differences seen.

None of the differences in adverse birth outcomes between older fathers and the 20 to 29-year-old reference group were statistically significant. This suggests that it would be incorrect to draw any conclusions from this study regarding older fathers and the health of their offspring.

Where did the story come from?

Dr Xi-Kuan Chen from the Department of Obstetrics and Gynecology at the University of Ottawa and other colleagues from Canada carried out the study. The study was supported by an award from the Canadian Institutes of Health Research. The study was published in the (peer-reviewed) medical journal: Human Reproduction.

What kind of scientific study was this?

In this retrospective cohort study, the researchers used data from population registries of 50 US states and the district of Columbia from 1995–2000. Detailed information on all live births and deaths of infants up to one year of age was compiled. This was linked to data on the parent’s background, characteristics, previous birth history, antenatal high-risk conditions, maternal lifestyle factors (such as smoking and alcohol consumption), information on the pregnancy, labour and delivery of the baby, and the baby’s gestational age, birth weight and health score at birth and up to one year after. Data on the mother’s, but not father’s, smoking and alcohol habits was also available.

The researchers looked for statistical links between the background data and a range of adverse birth outcomes. These included; the number of live babies born preterm, low birth weight, small babies for their length of gestation, a score of the baby’s distress at birth and finally, deaths of the babies at any time up to one year. A descriptive analysis of all the collected data took into account any confounding factors from the background of mothers and fathers that the researchers though might also influence the results.

What were the results of the study?

There were more than 23 million births in the 1995–2000 database. The researchers excluded twin births, children born to unmarried parents, mothers who had already had a child and mothers who were younger than 20 or older than 29. This left more than two million records for analysis, and insufficient data was available for about 150,000 women.

Compared with babies born to fathers aged 20–29 (the reference group), babies fathered by teenagers (less than 20 years old) had a statistically significant 15% increased risk of being born preterm and a 13% increased risk of having a low birth weight. There was a similar, small increase in risk for having a baby that was small for their length of gestation. The babies of teenage fathers also scored lower on measures of baby’s distress at birth and there were more baby deaths in the month after birth or up to one year in this group.

There was no association between advanced age of the father (over 40) and risk of adverse birth outcomes compared to the reference group.

What interpretations did the researchers draw from these results?

The researchers said that "teenage fathers carry an increased risk of adverse birth outcomes whereas advanced age of the father is not an independent risk factor for adverse birth outcomes”.

What does the NHS Knowledge Service make of this study?

This study collected data on the birth outcomes from a large number of pregnancies and sought to identify any links between these and the father’s age. This is a new area of research, but should be interpreted in the light of the authors’ own comments. It is unfortunate that more details regarding the fathers were not available to the researchers. As explained below, without more detail on their backgrounds, it is not possible to be sure that factors other than the father’s age were not more important. Bearing this in mind, they say that there may be three explanations for their findings.

• the sperm of younger men (under 20 years) may differ from those in older age groups and the differences may lead to the increased risk of adverse outcomes
• young fathers may came from more economically disadvantaged families
• lifestyle factors such as higher rates of smoking or greater alcohol intake may be more common in teenage fathers

All of these factors could explain why babies born to younger fathers were at greater risk of adverse birth outcomes. With the small differences shown in such a large study, it seems more likely that the strong association between socioeconomic factors and infant health accounts for the differences seen in this study, rather than the father’s age.