Determining Pregnancy Cohort
IBM® MarketScan® Commercial Database that includes health insurance claims data from across the US from 2007 to 2016 were used. A cohort of women aged 12–55 years with singleton pregnancies was identified from the IBM MarketScan Commercial Database inpatient admissions by using International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9) diagnosis codes (640-649 (Complications mainly related to pregnancy), 650, 652-659 (Normal delivery, and other indications for care in pregnancy, labor, and delivery), 660-669 (Complications occurring mainly in the course of labor and delivery), 670-677 (Complications of the puerperium), 678-679 (Other maternal and fetal complications), V27.0, V27.1, V27.9 (Outcome of delivery); only codes ending in ***.*1 and ***.*2 identifying delivery were used), ICD-10 codes (O80-O83 (Single delivery), Z37.0, Z37.1, Z37.9 (Outcome of delivery), O60.1*X0, O60.1*X1 (Preterm labor with preterm delivery), O42.1 except O42.111, O42.1 except O42.111, O42.9 except O42.911 (Premature rupture of membranes)(Marić et al. 2019)), diagnosis-group related (DRG) codes (765-766 (Cesarean section with/without complication or comorbidity), 767 (Vaginal delivery with sterilization and/or D&C), 768 (Vaginal delivery with O.R. procedure except sterilization and/or D&C), 774-775 (vaginal delivery with/without complications)) and procedure codes (72.** (Forceps, vacuum, and breech delivery), 73.22, 73.59, 73.6 (Other procedures inducing or assisting delivery), 74.0-74.2, 74.4, 74.99 (Cesarean section and removal of fetus)) as previously described in Marić et. al.8 Given the differences in sPTB risk between singleton and multiple gestations, deliveries with codes for multi-fetal pregnancies (ICD-9 codes 651, V27.2-V27.7, V-91; ICD-10 codes O30) were excluded. To avoid dependencies for women with multiple pregnancies, only the first pregnancy recorded in the IBM MarketScan Commercial Database was included. We excluded molar and ectopic pregnancies (ICD-9 codes 630 and 633; ICD-10 codes O00 and O01; DRG code 777), spontaneous and induced abortions (ICD-9 codes 632, 634-637; ICD-10 codes O03), complications following abortion, ectopic or molar pregnancies (ICD-9 code 639; ICD-10 codes O04 and O08), and abortions with/without D&C (DRG codes 770 and 779).
Gestational age (GA) at delivery was estimated by using the algorithm described in Marić et. al.8 Briefly, the algorithm is based on codes for the number of completed weeks of gestation (ICD-9 codes 765.2, 766.21, 766.22; ICD-10 codes Z3A) and codes for preterm labor with preterm delivery (ICD- 9644.21; ICD-10 codes O60.1*X0), identified directly from women’s records when present or, otherwise, from infants’ records. To obtain GAs from infant records, single liveborn infants were identified (ICD-9 codes V30 and V39; ICD-10 codes Z38.0-Z38.2) and linked to their mothers. Women without a GA code either in their own or their infant record, were assigned a GA of: 1) 34 weeks if they were identified to have sPTB, as described below, or 2) 39 weeks, otherwise.