Table 1 Baseline Characteristics of the Mothers of Mixed-Gender Twins.

From: Gender Differences in Infant Mortality and Neonatal Morbidity in Mixed-Gender Twins

Maternal characteristics
Maternal Race (n, %)
 White 84,215 (78.0)
 Black 20,382 (18.9)
 Others 3441 (3.2)
Maternal age (Mean, SD) 29.5 (6.0)
Maternal smoking status during pregnancy (n, %)
 No smoking 78979 (73.1)
 Smoking 9867 (9.1)
 Unknown 19192 (17.8)
Maternal education (n, %)
 <12y 15,162 (14.0)
 12–15y 56,540 (52.3)
 >15y 34,583 (32.0)
 Unknown 1753 (1.6)
Marital status (n, %)
 Married 78,599 (72.8)
 Unmarried 29073 (26.9)
 Unknown 366 (0.3)
 Weight gain during pregnancy (Kg) 16.6 (7.2)
Parity
 0 42,798 (39.6)
 ≥1 65,240 (60.4)
 Pregnant complications1 (n, %) 19,949 (18.5)
 Labor complications2(n, %) 14,892 (13.8)
Methods of delivery (n, %)
 Cesarean section 68,256 (63.2)
 Vaginal 27989 (25.9)
 Vaginal to Cesarean section3 4714 (4.4)
 Unknown 7079 (6.6)
  1. 1Defined as one or more of the following diseases or conditions: anemia (HCT <30 or Hb <100 g/L), cardiac disease; acute or chronic lung disease, diabetes, hydramnios or oligohydramnios, hemoglobinophathy, chronic or pregnancy-associated hypertension, and renal disease. 2Defined as one or more of the following diseases or conditions: premature rupture or membrane >12 hours, abruption placenta, placenta previa, precipitous or prolong labor, dysfunctional labor, seizure during labor, cephalopelvic disproportion, and excessive bleeding. 3The first babies were vaginal delivery and the second ones by cesarean section.