Professor Offenbacher will present his findings at a conference on "The Problem with Prematurity" on Monday.

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"I could have coped with them dying from prematurity, as that would have been nobody's fault, " she said.

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He said the cause of their deaths was "complications in extreme prematurity" but he could not say they died from natural causes.

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He died of extreme prematurity, according to the city medical examiner's office.

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The change is based on research indicating the high levels may contribute to a condition called retinopathy of prematurity, a leading cause of blindness.

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Children conceived by in vitro fertilization and other technologies have higher rates of prematurity and low birth weights, conditions associated with behavioral and developmental problems.

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He died of extreme prematurity, the city medical examiner's office said.

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He told the BBC News website the relationship between hCG and production of the muscle relaxant was still not completely clear, and the causes of prematurity were complex.

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Dr Joy Lawn, of Save the Children, who is part of the Born Too Soon initiative that seeks to cut prematurity rates, said: ""Our analysis shows that the current potential for preterm birth prevention is shockingly small.

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