Relationship between historical developments in the percentages of low birthweight and fetal mortality in Spain.

OBJECTIVE: To document the differences in low birthweight in different population subgroups and to analyze its possible relationship with fetal mortality rates in Spain.

METHOD: We used the microdata on live births and stillbirths since 1975 to 2020 from the Spanish National Statistics Institute to examine differentials in delivering a low birthweight controlling for different sociodemographic variables and to determine the relationship between low birthweight and fetal mortality. No statistical analysis was necessary beyond the calculation of percentages and rates.

RESULTS: The data at our disposal for Spain confirm the historical increase in the incidence of low birthweight and allow us to link trends in low birthweight with a decrease in the fetal mortality rate. When fetal mortality is high, the incidence of low birthweight is low, given that a natural selection effect takes place.

CONCLUSIONS: The surprising historical increase in the incidence of low birthweight in Spain can be explained by the improvement in mortality rates. As more fetuses of lower gestational age are born alive, cases of low birthweight also increase.