The World Health Organization defines normal term delivery as between 37 weeks and 42 weeks. (Ibraheem Abu Mustafa/Reuters)

Poor women are more likely to give birth to a very pre-term baby compared with wealthier women despite both groups receiving equal neonatal care, a British study suggests.

The findings show the need to better understand the link between socioeconomic status and pre-term birth, the researchers said in Wednesday's online issue of the British Medical Journal.

In the study, researchers from the University of Leicester tracked 7,449 very pre-term infants who were born between 1998 and 2007 in England in the area that used to be known as Trent region.

The infants were tracked from the onset of labour until they were discharged from neonatal care. The team calculated a deprivation score for each infant based on postal code data.

Mothers from the most deprived areas were nearly twice as likely to give birth to a very pre-term infant at less than 33 weeks of gestation compared with mothers from the least deprived areas. The WHO defines normal term delivery as between 37 weeks and 42 weeks.

There were also nearly twice as many deaths due to very pre-term birth in the most deprived areas.

But among very pre-term infants, survival rates and neonatal care — such as length of stay and use of ventilation — showed little variation across all deprivation scores. That suggests that although socioeconomic inequalities existed, deprivation did not seem to be a barrier to receiving neonatal care.

The higher burden of deaths and increased neonatal care costs for very pre-term infants in deprived areas is probably on account of those areas having higher incidence of pre-term labour and not because of differences in severity, the journal's editors said.

"It seems highly likely that such work could lead to public health strategies that would reduce the costs not only of neonatal care but also attached to the long-term health problems suffered by some of these babies," David Field, a professor of neonatal medicine, and his co-authors concluded.

In a journal commentary accompanying the study, Ron Gray from the University of Oxford and Marie McCormick from Harvard School of Public Health said there is unlikely to be progress in preventing pre-term birth "without a better understanding of the role of socioeconomic factors."

The study did not look at factors such as smoking, ethnicity, or history of previous preterm birth.

The researchers also cautioned that the findings came from one region that represents one in 12 U.K. births and may not necessarily be extrapolated to other places.