Determinants of birth weight in Portugal: 1988 to 2011

The objective of this paper is to analyse temporal birth weight variation, its relationship to the frequency of premature births in Portugal, and the influence of native and immigrant mothers' characteristics as well as to determine the possible existence of a pattern of temporal change in birth weight in the Iberian Peninsula as a whole. Individual mother-child data from the Portuguese National Institute of Statistics regarding live births (N = 2,661,542) permitted an analysis, for the first time, of weight at birth in Portugal from a bio-demographic perspective. The results obtained show that from 1988 to 2011 there was a gradual decline in the average weight at birth in Portugal that may be related to shifts in the duration of gestation. An initial rapid decline in the relative frequency of post-term births took place, followed by small variations from 1995 on. Logistic regressions indicated a pattern unaffected by maternal origin or the sex of the newborn. With regard to weeks of gestation, the odds values obtained were < 1 when the reference category was < 28 weeks. For this factor, no significant differences were found in relation to the mother's origin. Portuguese mothers over 35 years were associated with a higher incidence of low birth weight. Regardless of maternal origin, being a newborn of parity 1, and with the mother not in a couple, resulted in unfavourable outcomes with regard to low birth weight. On the other hand, long gestation periods and having secondary or university education constituted a protective factor.