to estimate incidences of major complications, such as bronchopulmonary dysplasia, death, and delivery room resuscitation among extremely preterm and extremely low birth weight infants in Northern China

to estimate incidences and risk factors of bronchopulmonary dysplasia, death, and delivery room resuscitation among extremely preterm and extremely low birth weight infants in Northern China

Inclusion Criteria:

1. born between January 1, 2015, and December 31, 2018;
2. a birth weight <1000grams or a gestational age <28 weeks;
3. admitted within 72 hours after birth.

Exclusion Criteria:

No medical history in the electronic record

infants with a birth weight <1000grams or a gestational age <28 weeks;