Preterm delivery and calculation of survival rate below 28 weeks of gestation

To estimate the number of still- and live-births below 28 wk of gestation in a 2-y period in Denmark, and to determine how the calculated survival rates vary using three different denominators: 1) the number of infants actively treated in the Neonatal Care Units, NNUs, 2) the number of all live-born infants, and 3) the number of all births, including stillbirths.

The number of stillbirths and live-born infants at 20 to 27 wk of gestation was estimated over a 2-y period in Denmark. Data on live-born infants and survival rates were obtained from a national prospective study and the National Births Register. The number of women hospitalized owing to spontaneous abortion was obtained from The Danish National Health Register; the number of stillbirths was calculated from these data.

The estimated number of both still- and live-births was similar in every week from 20 to 27 wk of gestation; approximately 1 per 1000 live-births at each gestational week. At from 23 to 25 wk of gestation the overall chances of survival varied significantly depending on the denominator: actively treated infants 46% (CI 95%; 37%-54%), all live-born infants 34% (CI 95%; 27%-41%), all births including stillbirths 14% (CI 95%: 11-17%).

The chances of survival for the intrauterine fetus below 26 wk of gestation are significantly lower than the reported survival rates from the NNUs. The low percentage of fetuses delivered before 26 wk of gestation classified as live-born renders the reported birth-rates sensitive to differences in the way "live-born" is interpreted.