Association between birth weight and adult disorders in twins: validity of self-reported birth weight

Validity of Self-Reported Birth Weight: Results from a Norwegian Twin Sample

The association between birth weight and later life outcomes is of considerable interest in life-course epidemiology. Research often relies on self-reported measures of birth weight, and its validity is consequently of importance. We assessed agreement between self-reported birth weight and official birth records for Norwegian twins born 1967–1974. The intraclass correlation between self-reported birth weight and register-based birth weight was 0.91 in our final sample of 363 twins. It could be expected that 95% of self-reported birth-weight values will deviate from official records within a maximum of +446 grams and a minimum of −478 grams — around a mean deviation of 16 grams. Self-reported birth weight had a sensitivity of 0.78–0.89 and a positive predictive value of 0.59–0.85, and an overall weighted kappa of 0.71. We further assessed agreement by conducting two linear regression models where we respectively regressed self-reported birth weight and register-based birth weight on adult body mass index, a known association. The two models were not significantly different; however, there were different levels of significance in parameter estimates that warrant some caution in using self-reported birth weight. Reliability of self-reported birth weight was also assessed, based on self-reports in another sample of twins born 1935–1960 who had reported their birth weight in two questionnaires 34 years apart. The intraclass correlation was 0.86, which indicates a high degree of reliability. In conclusion, self-reported birth weight, depending on context and age when birth weight was reported, can be cautiously used.

Newborn infant characteristics and risk of future rheumatoid arthritis: a twin-control study

Low birth weight has been proposed as a risk factor for rheumatoid arthritis (RA). The twin-control study design provides an opportunity to investigate the significance of potential prenatal determinants for adult morbidity by accounting for maternal characteristics and early environmental and genetic factors. We investigated the association between birth weight and RA in a sample of 42 twin pairs discordant for rheumatoid arthritis in which valid information on birth weight, birth length, and order was available from midwife records. Difference plot and conditional logistic regression were used to investigate the relationship between RA and birth weight or birth order adjusting for birth length and sex. The intra-pairwise birth weight differences, i.e., RA twin minus co-twin, ranged from -750 to 1,100 g, mean 78 g (95 % CI -13 to 70), 146 g (95 % CI (-36 to 329) in monozygotic, 32 g (95 % CI -90 to 154) in dizygotic, same sex and 69 g (95 % CI -122 to 260) in dizygotic, opposite sex twin pairs. The odds ratio for birth weight as risk factor for RA was 1.00 (95 % CI 0.997-1.003) when adjusting for birth length, birth order, and sex, irrespective of ACPA status. The odds ratio for developing RA as first born twin was 2.33 (95 % CI 0.97-5.60) when adjusting for birth length, birth weight, and sex, irrespective of ACPA status. In this twin-control study, birth weight was not associated with the development of RA in adult life. Being born first may predispose to RA.

A comparison of the early growth of twins and singletons

BACKGROUND

It is well known that the birth weight of twins is less than that of singletons, but there is less information about how birth lengths compare and how twins grow in the postnatal and early childhood period. The implications of the shorter duration of gestation of twins in comparing their postnatal progress with that of singletons are seldom taken into consideration.

AIMS

The study aimed to compare the growth of healthy twins and singletons over the first 4 years of life.

SUBJECTS AND METHODS

Birth length data on English twins born between 1990 and 1997 were used to produce reference values for gestational ages from 33 to 40 weeks. Follow-up data of weights and lengths up to the equivalent of 40 weeks' gestation are reported on some of these infants who were born pre-term. On the basis of questionnaires, the growth of twins in terms of weight, length/height and head circumference has been followed up to 4 years of age.

RESULTS

Mean birth length of twins showed a fall-off when compared with singletons at comparable gestational ages, which was more marked in boys than girls and evident earlier. Extrauterine growth of twins born pre-term was comparable to that of adequately nourished twins remaining in utero until term. Much of the smallness of twins at birth was because of their shorter gestation. Subsequently singletons gained weight faster. Lengths/heights showed no deficit compared with singletons. Head circumferences of twins were, however, smaller than singletons throughout.

CONCLUSIONS

The early growth of normal twins differs markedly from that of singletons. It is important to recognize that singleton reference values do not correctly reflect the growth of twins.