Health: Children all in good time

Siblings are fine, but could there be problems for brothers and sisters born too close together? By Raj Persaud

Most couples planning a family wonder how long a gap to leave between their children - assuming that they want more than one. As women increasingly postpone childbirth until later in life, there is evidence that they are trying to have these late pregnancies close together, to ensure that the family is complete before the biological clock stops ticking.

But could close spacing between children be medically and psychologically damaging? Research published in the New England Journal of Medicine should give pause for thought to couples planning to squeeze a family into a few years.

In the largest study of its kind ever conducted, 173,205 births in Utah during the last seven years were examined by Dr Bao-Ping Zhu and colleagues from the Michigan Department of Community health, investigating what is the optimal spacing between children in order to minimise their medical risks. The study found the best interval between pregnancies, for preventing adverse perinatal events such as low birth weight, excessively small babies and pre-term delivery, was between 18 and 23 months.

One theory to account for this finding is that after each pregnancy uterine blood flow increases temporarily, but it probably declines gradually over time. So this, and other foetus growth-supporting adaptations to pregnancy in the mother's body, could benefit the next baby, but only if not too long a gap is left between births.

Too short a gap is a risk factor for perinatal problems because the stress of looking after an excessively dependent baby is thought to place such a psychological stress on the mother's body that it interferes with the growth of the next foetus. It is already known that babies born to mothers in emotional turmoil are more prone to complications. They are even more likely to grow into adults who suffer from psychiatric disorders, including schizophrenia, and this may be the result of the lowered uterine blood flow.

Another theory is that each pregnancy depletes the mother's nutritional resources, which then need adequate time between pregnancies before they can be restored. But while physicians may argue that the medical risks mean mothers should optimally not space babies more widely than 23 months apart, psychologists suggest that a birth spacing of at least two years may be preferable, in order to enhance the psychological future of the children.

When babies are closely spaced, child-rearing obligations dominate, ensuring that parents give less undivided concentration to any one child. How much attention a child gets in the early years, from adults and older children, has repeatedly been found to predict future intelligence. For example, the fact that the Japanese have the highest national average IQ has been partly attributed to their child-rearing practice of holding their babies much more than we do in the West.

More support for the importance of birth spacing in determining IQ comes from the extreme case of children with no birth gap at all, and twins and triplets do indeed score lower on IQ tests. But there is a debate as to whether this is due to intrauterine problems of several babies competing for nutrients from an overworked placenta, or the stretching of parental attention once the children are born. Brian Powell, a sociologist at Indiana University, and a world authority on the effects of spacing on subsequent behaviour, has calculated that among high school students with two siblings, one whose siblings are born within two years is more than 50 per cent more likely to drop out and have poor exam performance than a student whose siblings are more than two years older or younger.

The classic argument against these spacing theories is that poorer families tend to have more closely spaced children, and that it is the poverty that is producing these negative effects. But Powell and colleagues have taken this into account in their research, and found the negative effect of spacing is a factor separate from socio-economic status.

Philippe Rushton, a Canadian psychologist, goes much further, suggesting that birth spacing influences altruism and respect for the law in children. The contention is that siblings born too close together grow up constantly competing with each other for parental resources. Growing up in a competitive, jealous atmosphere may foster an ungenerous attitude towards others, hence less altruism.

Several children of similar age will also more easily escape parental surveillance than those with widely spaced births, who find their older siblings assisting in the parental monitoring. A less respectful attitude to authority and rules should therefore be characteristic of closely spaced children. However, Rushton has tended to produce evidence of these differences only between societies that may have characteristic birth-spacing sequences, and such contrasts in altruism and law-abiding behaviour could arise for other reasons.

Yet Elizabeth Gibbs and her psychologist colleagues at the University of Vermont did indeed find that widely spaced children took more turns when playing together, and they seemed to have more fun with each other than narrowly spaced siblings. Psychologists tend to agree that siblings born closer together have more intense relationships with each other than average, often characterised by more arguments.

The danger of being born far apart is that older siblings miss out on much of the growing up of their younger siblings. But there is an advantage in very wide spacing for the intellectual development of the youngest child. IQ in siblings follows a declining gradient with every subsequent birth. This phenomenon is usually explained by diminishing parental attention per infant, with each child's arrival.

But a landmark study of birth order and IQ conducted on 350,000 Dutch young men in the Seventies found that as family size increased beyond about five children, the intelligence of the youngest child began to rise. This odd finding is explained by the idea that once you get to about five children in a family, the first child has reached an age at which it can add to the intellectual atmosphere in the home. This is in comparison with the situation of the youngest child in a smaller family.

The presence of older children, who will themselves give a baby attention without being so dependent on parents as to take their concentration off the new child, means that an old enough sibling adds to, rather than detracts from, the intellectual stimulation a new baby enjoys. This logic, carried to its extreme, suggests that if a couple space two children 15 years apart (highly unusual, but not unheard of), the second child enters a home with a climate of potential stimulation even higher than that encountered by most first-borns.

While all the evidence so far points to a two-year gap between children being the best for psychological reasons, and the medical "window" is 18 to 23 months, no research has yet been conducted on parents who are aware of these birth-gap issues. Perhaps the real benefit of such research is that parents becoming aware of the findings could make a conscious effort to compensate medically and psychologically for the inherent problems of short spacing, and could then find that the adverse effects apply much less powerfully to their families.

The writer is a consultant psychiatrist at the Maudsley Hospital in south London