Petteri Hovi and colleagues evaluate skeletal health in 144 adults born preterm with very low birth weight and show that as adults these individuals have significantly lower bone mineral density than do their term-born peers.
Very-low-birth-weight (VLBW, <1,500 g) infants have compromised bone mass accrual during childhood, but it is unclear whether this results in subnormal peak bone mass and increased risk of impaired skeletal health in adulthood. We hypothesized that VLBW is associated with reduced bone mineral density (BMD) in adulthood.
Methods and Findings
The Helsinki Study of Very Low Birth Weight Adults is a multidisciplinary cohort study representative of all VLBW births within the larger Helsinki area from 1978 to 1985. This study evaluated skeletal health in 144 such participants (all born preterm, mean gestational age 29.3 wk, birth weight 1,127 g, birth weight Z score 1.3), and in 139 comparison participants born at term, matched for sex, age, and birth hospital. BMD was measured by dual energy X-ray absorptiometry at age 18.5 to 27.1 y. Adults born with VLBW had, in comparison to participants born at term, a 0.51-unit (95% confidence interval [CI] 0.28–0.75) lower lumbar spine Z score and a 0.56-unit (95% CI 0.34–0.78) lower femoral neck Z score for areal BMD. These differences remained statistically significant after adjustment for the VLBW adults' shorter height and lower self-reported exercise intensity.
Young adults born with VLBW, when studied close to the age of peak bone mass, have significantly lower BMD than do their term-born peers. This suggests that compromised childhood bone mass accrual in preterm VLBW children translates into increased risk for osteoporosis in adulthood, warranting vigilance in osteoporosis prevention.
Please see later in the article for the Editors' Summary
Most pregnancies last 40 weeks but some babies arrive earlier than expected. Sadly, babies born before 37 weeks of pregnancy—premature babies—are more likely to die than full-term babies, although recent improvements in neonatal care have increased their chances of survival. Premature babies also often have serious long-term health problems, particularly those born before 32 weeks of pregnancy. Such extremely premature babies have poorly developed internal organs and are usually very small—babies whose birth weight is less than 1,500 g are called very-low-birth-weight (VLBW) babies; the average full-term birth weight is about 3,500 g. Furthermore, their bones are not as well developed as those of full-term babies. The human skeleton initially consists of a soft fibrous material called cartilage. This is gradually transformed into bone by a process called bone mineralization. The last third of pregnancy is a crucial period for bone mineralization although the process continues throughout infancy and childhood. Thus, VLBW babies often have subnormal skeletal mineralization and their accrual of bone mass during childhood is frequently compromised.
Why Was This Study Done?
It is not known whether the childhood bone deficits of VLBW babies persist into adulthood because the first generation of these infants not to die soon after birth is only just reaching adulthood. Peak bone mass is reached in early adulthood (bone mass begins to decrease from the age of 35 years onward) and is an important indicator of whether an individual will develop osteoporosis (thinning of the bones) and be susceptible to bone fractures later in life. If adults with VLBW (about 1% of live births in high-income countries are now VLBW births) do have a subnormal peak bone mass and reduced bone mineral density (BMD), they may be able reduce their risk of developing osteoporosis by eating a healthy diet and exercising regularly. In this study (part of the Helsinki Study of Very Low Birth Weight Adults), the researchers investigate the skeletal health of people who were born with VLBW in the Helsinki area between 1978 and 1985.
What Did the Researchers Do and Find?
The researchers compared the skeletal health of 144 young adults who were born prematurely with VLBW and subnormal BMD with that of 139 age- and sex-matched individuals who were born at term. They measured the BMD of the participants (average age 22.6 years) using “dual energy X-ray absorptiometry” and determined a “Z score” for the spine in the lower back (the lower lumbar spine) and the hip (two sites that are routinely examined in assessments of skeletal health). Z scores indicate whether an individual's BMD is significantly different from the average BMD of healthy age- and sex-matched people; in this study, reduced BMD was defined as a Z score of −1.0 or less. The researchers found that adults born with VLBW had an average Z score of −0.51 at the lower lumbar spine and −0.56 at the hip when compared with the adults born at term. Furthermore, 44% of the VLBW participants but only 26% of the term-born participants had a lumbar spine Z score of −1.0 or less. Adjustment for the shorter height of the VLBW participants slightly reduced these differences in BMD but the differences remained statistically significant.
What Do These Findings Mean?
These findings show that, when studied close to the age of peak bone mass, young adults born with VLBW have a significantly lower BMD than their term-born peers and a 2-fold greater risk of having a lumbar spine Z score of below −1.0; a unit decrease in Z score approximately doubles the risk of bone fractures. Because BMD measurements were only taken at one age, it remains possible, however, that the BMD of the VLBW adults might eventually match that of their full-term peers. Recently born VLBW babies still have a lower than average BMD during their childhood, note the researchers, even though their care has changed since the people included in this study were born. Thus, these findings suggest that people who were VLBW infants should be encouraged to eat food rich in vitamin D and calcium and to do regular weight-bearing exercise throughout their lives to improve their bone health and reduce their risk of developing osteoporosis.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.1000135.
The March of Dimes, a nonprofit organization for pregnancy and baby health, provides information on preterm birth (in English and Spanish)
The Nemours Foundation, another nonprofit organization for child health, also provides information on premature babies (in English and Spanish)
MedlinePlus provides links to other information on premature babies and to information on osteoporosis (in English and Spanish)
The US National Institute of Arthritis and Musculoskeletal and Skin Diseases and the UK National Health Service also provide detailed information on all aspects of osteoporosis
Further details about the Helsinki Study of Very Low Birth Weight Adults are available