Schizophrenia Research Blog

Risk from short birth intervals

Association between short birth intervals and schizophrenia in the offspring.

Smits L, Pedersen C, Mortensen P, van Os J.
Schizophr Res. 2004 Sep 1;70(1):49-56.

Background: Conceiving immediately after delivering a baby has been found to increase risk for premature births, maturity problems and birth defects in the child. Some have hypothesized that this increased risk may be due to incomplete restoring of nutrients such as folic acid, vitamins B6 and B12 in the mother at the time of conception. These nutrients usually take up to 1 year to return to recommended levels. Since there may be a link between prenatal deficiencies of these nutrients and schizophrenia, this study looked at the relationship between the length of the preceding birth interval and the risk of schizophrenia in the offspring.

Method: This was a Danish study that collected information from a huge number of individuals - over 1.4 million Danish-born individuals and their families - and linked it to data from the Danish Psychiatric Central Register which contained data on all admissions to Danish psychiatric inpatient facilities over a certain time period. Using this database, they calculated birth order, size of family and birth interval to nearest older and younger sibling and did various other analyses.

Results: From their group, they found that a total of 5095 persons were diagnosed with schizophrenia. They found that schizophrenia risk changed depending on the birth interval. Those born after long birth intervals (27 months and longer) had the lowest chance of having schizophrenia, while those with moderately short birth intervals (15 to 20 months) showed highest rates. Contrary to what you would expect, they found that there was less risk in those born after very short birth intervals (<15 months) or after intermediate birth intervals (21 to 26 months). All these associations were independent of history of mental illness in a parent or sibling, season of birth, parental age, birth order, level of urbanization at birth, or calendar year.

Interpretation and Limitations: This study suggests that there is a link between birth spacing and the risk of schizophrenia in the child. Individuals born after birth intervals of up to 26 months, and predominantly those born within 15 to 20 months after their preceding sibling, had higher risks of developing schizophrenia than those born after birth intervals of 45 months or more. However, they had an interesting finding where the shortest interval did not result in the greatest risk, as you’d expect if the hypothesis is true about incomplete restoration of maternal nutrients of B-vitamins such as folic acid and vitamin B6. The authors try to explain this contradiction by suggesting that the shortest birth intervals may have occurred in non-lactating women who tend to have more favorable folic acid levels than lactating women. But this is just a hypothesis and needs to be researched further. Also, while this study focused on those with at least one older sibling, they also looked at data from first-born individuals and found that the highest rates associated with short birth interval was about the same as being an only child.

There are several limitations to this study. Since the authors did not have information on duration of pregnancy, they used birth intervals as estimation for inter-pregnancy interval – which might result in overrepresentations of things like premature births in the shortest birth intervals. Since prematurity may also be a risk factor for schizophrenia, this may have biased the results. Also, this study did not look at the effects of some other hypothesized environmental risk factors of schizophrenia possibly associated with birth interval such as maternal stress during pregnancy, later infections or others such as smoking, socio-economic status, planned pregnancy etc. Additionally, since this was a Danish study, their sample may not be representative of other countries with different levels of nutritional problems in the population.

Support: This study was supported by the Stanley Medical Research Institute, the Maastricht Care and Prevention Research Institute (CAPHRI) and the European Graduate School for Neuroscience (EURON) and the National Centre for Register-based Research is funded by the Danish National Research Foundation.

Click here to find this article on PubMed

Posted by Farzin at January 12, 2005 03:00 PM | TrackBack