According to the paper, "The data include the infant’s month of birth, and a clinical estimate of gestation in weeks, which we use to estimate a month of conception."
On miscarriages, "We interpret these data with caution because they come from a subset of one state, and because fetal deaths are under-reported. Nevertheless, the data provide no evidence of an increase in
miscarriages leading up to recessions that is anywhere near the magnitude required to explain a significant portion of the observed decrease in births." They similarly argue that abortions aren't significant enough to explain the reduction in births.
On miscarriages, "We interpret these data with caution because they come from a subset of one state, and because fetal deaths are under-reported. Nevertheless, the data provide no evidence of an increase in miscarriages leading up to recessions that is anywhere near the magnitude required to explain a significant portion of the observed decrease in births." They similarly argue that abortions aren't significant enough to explain the reduction in births.