Professor Brenda Eskenazi and colleagues studied data from birth certificates of 1,660,401 babies born in New York between January 1996 and December 2002. They divided the babies into those born in New York City (NYC) – whose mothers would, therefore, have been living closest to the disaster zone – and those born in “upstate” New York, which they defined as anyone living outside NYC, including Nassau, Suffolk and Westchester Counties.
When they compared data from babies born in the week before the disaster with those born in the week after in NYC, they found a shift in the distribution of low birth weights (LBW), with a higher proportion of babies being born weighing less than 2,000g. “Normal” birth weight is considered to be above 2,500g.
Prof Eskenazi said: “In New York City in the week after 9/11 we found there was a slightly increased risk (44%) of new-born babies weighing below 1,500g and a 67% increased risk of babies weighing between 1,500g and 1,999g compared with the three weeks before the disaster. There were no statistically significant changes in any LBW category in upstate NY, or in babies being born preterm in either location.”
However, there was non-statistically significant evidence that the increase in LBW in the first week after 9/11 was due to babies being born early. Gestational age data were provided to researchers in relatively broad categories which limited a more detailed investigation. In addition, the researchers were hampered by unreliable data on gestational age, based on last reported menstrual period, and the lack of medical reasons for LBW. “I think that the measures of gestational age on birth certificates are often not accurate, and the associations we are seeing with birth weight likely reflect shorter gestation and earlier births, rather than a reduction in foetal growth,” explained Prof Eskenazi, who is Professor of Maternal and Child Health and Epidemiology and Director of the Center for Children’s Environmental Health at the School of Public Health, University of California, Berkeley, USA.
Overall, moderately pre-term births (32-36 weeks) were reduced in both areas for the first month post-9/11. “We think that probably the most vulnerable foetuses were miscarried or born prematurely in that first week after 9/11, leaving behind those who were the strongest,” said Prof Eskenazi. “Another possibility is that one response to stress might be to ‘hold onto’ the foetus and to deliver later.”
At Christmas/New Year and around April/May, the researchers found another increase in the odds of LBW. These periods were when, at the time of the disaster, the babies would have been in either their first or second trimester of gestation or being conceived.
In NYC there was a 36% increased risk of babies being born weighing less than 1,500g in December 2001, and a 28% increased risk in January 2002. In upstate NY, there was a similar peak around the new year, with a 46% increased risk in January of LBW less than 1,500g. In the April/May period in NYC there was a 29% increased risk, and a 32% increased risk in upstate NY.
“We think that the increased incidence in low birth weights is mainly due to stress-initiated early deliveries. We had hypothesised that women further away from the disaster might have less stress associated with the event. We observed immediate effects in NYC, but longer-term effects both in NYC and upstate,” said Prof Eskenazi. “This may indicate that higher levels of stress are necessary to induce acute effects on birth outcome, such as early delivery with the consequent low birth weight, but that, in the longer term, women in both locations suffered stress as a result of the disaster and this is reflected in the later peaks in low birth weights.”
She said it was difficult to explain why these later peaks occurred. “It might be directly related to the disaster having occurred early in gestation, perhaps when the foetus was more susceptible to the effects of stress. Another hypothesis is that the Christmas and New Year holiday was a particularly emotional time after the disaster. The increase in very low birth weights (less than 1,500g) 33-36 weeks after September 11th suggests that exposure around the time of conception may also impact birth outcomes, although the exact mechanisms remain unknown.”
Prof Eskenazi concluded: “We have thoroughly reviewed the literature on preterm birth and low birth weight, and there is, thus far, a paucity of hard data to support the anecdotal information that women are more likely to have a spontaneous preterm birth immediately after a stressful event. Although we can't say for sure that our findings of increased births weighing less than 2,000g immediately after the events of September 11th are directly attributable to preterm delivery, we think that our results support this hypothesis and the paper supports the idea that stressful community events can impact the health of the foetus.”-European Society for Human Reproduction and Embryology