The rate of twin pregnancies in China has increased by 50 percent over the past decade and now stands at about 3 percent. This is significantly higher than the European and American countries. There are significant differences in birth weight of some newborns, that is, birth weight-discordance in twin pregnancies (BWDT). BWDT was significantly associated with fetal death, preterm birth, neonatal death, and neonatal complications. Existing studies mainly focus on the perinatal outcomes of mothers and infants with BWDT, or are reported from small samples in developed countries. The investigators are aware of the need to further explore the occurrence of birth weight inconsistency and adverse neonatal outcomes in China with a large sample. Through retrospective study design, the investigators will collect clinical data of live twin births at multiple study sites, investigate the incidence of BWDT, and analyze its adverse outcomes.

The investigators adopted a multicenter retrospective case-control study design and enrolled live twin births delivered from 2018 to 2020 in 22 hospitals in 12 provinces in China. Children with congenital and hereditary diseases, such as inherited metabolic diseases, complex congenital heart disease, and digestive tract malformations, were excluded from the study. The investigators collected the following information: (1) General information: maternal age, year of delivery, number of delivery, mode of conception, mode of delivery, chorionic nature, education level, newborn sex; (2) Maternal risk factors associated with intrauterine development: induced abortion ≥2 times, placental disease, uterine tumor, gestational diabetes mellitus, gestational hypertension disease, intrahepatic cholestasis, connective tissue disease, hypothyroidism, and chorionic amnitis; (3) Infant birth outcome: gestational age, birth weight, birth length, birth head circumference, very low birth weight infants, ultra-low birth weight infants, the Apgar score, whether transferred to neonatology department. (4) Information of hospitalization during the neonatal period: Pulmonary surfactant use, duration of oxygen therapy, neonatal complications (i.e., pulmonary hemorrhage, BPD, NEC, PDA, ROP, PVL, IVH, anemia, infection, etc.), feeding status (i.e., duration of milk opening, duration of full feeding, feeding style and feeding materials at full feeding, Time to recovery of birth weight), and discharge outcome (i.e., total hospitalization cost, length of stay, gestational age corrected for discharge, discharge status, discharge physique).

Inclusion Criteria:

live twin births

1. delivered from 2018 to 2020

Exclusion Criteria:

1. congenital and hereditary diseases
2. inherited metabolic diseases
3. complex congenital heart disease
4. digestive tract malformations

live twin births