Between January 8, 1996 and October 31, 1997, 19,507 infants were admitted to participating NICUs. Excluding those with congenital anomalies, there were 3,119 infants ≤30 weeks gestational age and 3,409 infants with birth weight <1,500 grams. Characteristics of the two groups are shown in Table . Of note, for infants <1,500 grams birth weight, 52% were born by caesarean section, 21% were outborn and 68% received antenatal steroids.
Characteristics of preterm and very low birth weight (VLBW) infants in the study cohort
Survival to NICU discharge
Survival to discharge increased from 45% for infants weighing <600 g at birth to over 95% for those with birth weight >1200 g (Fig ). There was a similar increase for gestational age groups with survival increasing from about 14% at 22 weeks to over 93% at 28 weeks and above (Fig ).
Gestational age specific survival.
Effect of gender on survival and morbidity
When analysed by gestational age groups, survival for male and female infants was not significantly different except for those born at 24 weeks gestation in whom survival for females was 17 (3, 29)% higher (Fig ). When analysed according to birth weight groups, survival for females in the 600–699 g group was 7 (6,28)% significantly higher than for males (Fig ). However, within many birth weight groups, females were significantly more mature than males and would therefore be expected to have higher survival as a consequence of gestational age (Tables and ). In contrast to survival, female infants had significantly (p < 0.05) lower incidence of chronic lung disease and severe intraventricular hemorrhage than male infants (Table ). The high incidence of SGA among infants born at <1,500 g reflects the inherent tendency to select SGA infants when birth weight criteria is used to categorise infants instead of gestational age.
Male/Female survival by gestational age.
Male/Female survival by birth weight.
Comparison of mean gestational age (GA) between male and female infants in each birth weight group (VLBW)
Comparison of mean birth weight between male and female infants in each GA group (Preterm)
Male/Female characteristics of preterm (≤30 weeks gestation) and very low birth weight Infant (< 1,500 g) groups
Multiple births and antenatal steroids
Of the 3119 infants born ≤30 weeks, 2,277 (73%) were singleton deliveries and 841 (27%) were the products of multiple gestation pregnancies (Table ). There was no significant difference in survival, illness severity or resource use between the two groups. Antenatal steroid use was 7(4,11)% higher in multiple gestation for VLBW babies, and delivery by cesarean section was 9(5,13)% more likely in multiple gestation pregnancies for VLBW. Antenatal steroid use was 7(3,11)% higher in multiple gestation for preterm babies, and delivery by cesarean section was 9(6,13)% more likely in multiple gestation pregnancies for preterm. A full or partial course of antenatal steroids was given to 70% of infants ≤30 weeks gestation and to 68% of infants between <1,500 g birth weight. The use of antenatal steroids was lower in the most preterm infants ≤24 weeks gestation.
Characteristics of singleton and multiple birth preterm infants
Actuarial survival curves are given in Figures , , , , for the first 60 days. Points on the curves give S(t); the estimated probability of a baby surviving to at least t days. Babies that were discharged from the NICU can not afford to be ignored as these were most likely to be the longer lived observations. Babies that were discharged from the NICU were therefore included as censored observations in the analysis. By comparing the slope of the curve (which gives the instantaneous risk of death), the highest risk of death is within the first 6 days. Risk of death is higher for smaller babies and babies with lower GA. Risk of death also decreases with time. Figure shows that actuarial survival is higher among females than males during the first 40 days, and narrows after that. Figures and give the probability of survival to discharge for an infant who has survived to a given day in the NICU, stratified by gestational age and birth weight respectively.
Actuarial survival curves stratified by gestation (weeks), by gender.
Actuarial survival curves by gestational age, with 95% CI.
Actuarial survival curves stratified by birth weight category; by Gender.
Actuarial survival curves by birth weight category.
Actuarial survival curves by sex.
Probability of survival to discharge (y-axis), for male and female infants surviving to a given day in the NICU (x-axis), stratified by gestation age.
Probability of survival to discharge (y-axis), for male and female infants surviving to a given day in the NICU (x-axis), stratified by birth weight (g).