Premature birth and long-term health

“Premature babies face lifelong health problems”, is the headline in_ The Independent_ . The article goes on to say that these findings are “threatening a public health crisis” due to the lifetime burden of problems for babies born prematurely including poorer health, a lower educational level, and being “less likely to have families of their own, and more likely to have offspring of their own born prematurely and with complications”, the newspaper adds.

The newspaper story is based on a Norwegian study of more than one million babies, which looked at their health from birth to adulthood. The findings of the study will come as no surprise to health professionals. A very large number of babies today are born prematurely (reportedly one in eight in the UK) and although rates of complications and infant death rates are known to be higher in this vulnerable population group, the majority of premature babies will go on to lead completely full, healthy and active lives that are no different from their full-term counterparts. Medical advances and improved healthcare (all babies in this study were born 20 to 40 years ago) will continue to ensure that all premature infants receive the best care and start in life possible.

Where did the story come from?

Dr Geeta Swamy and colleagues of Duke University Medical Center, North Carolina; Duke-NUS Medical School, Singapore; the University of Bergen and the Norwegian Institute of Public Health, Bergen, Norway carried out this research. No sources of funding were reported by the study authors. It was published in the peer-reviewed medical journal: Journal of the American Medical Association .

What kind of scientific study was this?

This was a cohort study in which the authors followed a large number of single-birth babies over time to see what effect prematurity (birth before 37 weeks of pregnancy) had upon survival rate, reproduction in adult life and premature birth in subsequent generations.

For their study, the researchers used the Medical Birth Registry of Norway, which has recorded all live and stillbirths in Norway since 1967. Between 1967 through to 1988, they identified 1,167,506 single live births and fetal deaths of babies born at 22 weeks of pregnancy or over and weighing 500g or more. The gestational age of all babies was estimated by date of last menstrual period and clinical examination at birth. The researchers used personal ID numbers to link data from the birth registry to the National Cause of Death Register and the Registry of Level of Education. They followed the entire group until 2002 to look at survival outcomes.

Babies born in the first 10 years of the study were also looked it in terms of educational and reproductive results up until 2004 (any abortions in the study group were not counted). For their analyses, the researchers grouped the babies into categories of: extremely premature (22 to 27 weeks), very premature (28 to 32 weeks), premature (33 to 36 weeks), full-term (37 to 42 weeks) and post-term (43 or more weeks). They looked at how the groups differed in terms of the infant, childhood and adolescent mortality rates; the proportion receiving high school or graduate education; and the reproductive outcomes, including subsequent premature birth rates and infant mortality. In their risk calculations, the researchers grouped the subjects by sex and adjusted for the possible confounding factors of year of birth and maternal age and education.

What were the results of the study?

Of the entire cohort, 5.2% of babies were born prematurely, with a slightly higher proportion of these being male. There were no differences in the mother’s or father’s age between premature and full-term babies; however, as expected, lower maternal education level, being a single mum, diabetes and pre-eclampsia were higher in the premature groups.

Rates of stillbirth, infant (less than one year old) and childhood (between age one and six years) mortality were highest in the extremely premature group. The percentage of infants that died before they were one year old was less than 1% for full-term babies increasing to 3.5% for boys and 3% for girls in the premature group, 25 and 20% respectively in the very premature group, and 75 and 70% in the extremely premature group.

The risk of early childhood mortality (between age one and six years) was also increased for all premature age categories in boys and for girls, apart from the middle “very premature” category in girls, for whom it did not reach statistical significance. The risk of mortality in late childhood (between age six and 13 years) was raised for extremely and very premature boys, but not significantly raised for girls. The rates of adolescent mortality were not significantly different between any premature category and full-term babies in boys or girls.

When the researchers looked at reproductive outcomes, they found that a smaller proportion of the group had had children themselves as the prematurity increased, from 68% of women and 50% of men in the full-term group down to 25% of women and 14% of men in the extremely premature groups. The rates of premature birth in the offspring were slightly greater among women who had been premature themselves, but were no different between premature fathers and full-term fathers.

There was much less difference between the groups in proportions receiving less than high school or graduate education.

What interpretations did the researchers draw from these results?

The authors conclude that in their Norwegian cohort, prematurity was associated with increased death rates around the time of birth and continuing into infancy and childhood and with lower reproductive rates in adult life.

What does the NHS Knowledge Service make of this study?

This is a very large and reliable quantity of data. However, the findings will come as no surprise to the medical profession and should not cause undue concern to parents of premature children.

• Mortality rates and complications are unfortunately higher among vulnerable premature babies and rates will increase with increasing level of prematurity. However, despite this, a very large number of babies today are born prematurely and the majority will go on to lead completely full, healthy and active lives that are no different from their full-term counterparts.
• The study has used gestational age to categorise all of the subjects and, as the authors themselves state, birth weight may have been a more reliable marker of extent of prematurity. However, ultrasound confirmation of gestational age was not available when this birth cohort was taken.
• As mortality figures have been obtained from a register, more detailed information on the causes of death is not provided. This may have been able to provide more of an indication of whether the cause could possibly have been attributed to prematurity rather than an unrelated cause (e.g. death as a result of a heart defect or a road traffic accident).
• The possible reasons behind reduced reproductive level among premature people are unclear from this study, and this requires further research. However, it should be noted that it was not possible to consider the entire cohort in this analysis, and that not all will have necessarily had the desire or opportunity to reproduce at the time of data collection.
• The study was conducted in Norway where healthcare systems and mortality rates cannot be assumed to be the same as elsewhere.

Premature births will continue to occur and little can be done to prevent them. However, medical advances and improved healthcare as time progresses (all babies in this study were born 20 to 40 years ago) will continue to ensure that all premature infants receive the best care and start in life possible.

Sir Muir Gray adds...

There is evidence that some of the very low birth weight children who survive have less than ideal health, but this has been known for years. Thanks to medical advances some of the children who do not have any health problems would have had problems if it had not been for the high quality care they had received in the special care baby unit.