Primary Exposure and Covariates
Our primary exposure was assignment to either the intervention or
standard of-care-arms of the study. Covariates of interest were factors
found to be associated with preterm birth(14) or low birth weight(15) in
previous research that could be imbalanced between the arms. Initial
covariates examined were age (≤25 years vs above 25 years), relationship
status (married/cohabitating vs not), education level (primary level or
less vs secondary level or higher), income (≤2000, >2000
Pula (~USD 160)/month, or unknown), nationality
(Motswana vs other), HIV status (positive vs negative), partner’s HIV
status (positive, negative, or unknown), parity (prior live birth vs
not), recorded history of prior preterm or low birth weight outcome
among normal singleton births (ever had a preterm birth or low birth
weight infant vs. no history of preterm or low birth weight births among
normal singleton births), treatment for STI symptoms between the first
and second study visits (STI treatment included azithromycin,
erythromycin, ceftriaxone, doxycycline, and metronidazole), proportion
of WHO recommended antenatal care visits achieved by gestational age at
delivery,(16) delivery mode (spontaneous vaginal delivery vs other), and
hypertension during pregnancy (defined as either a systolic pressure of
≥140 mmHg or diastolic pressure of ≥90 mmHg recorded at first antenatal
care visit or during the third trimester. We also examined any
hypertension vs none, as well as through a three-category variable:
“early” hypertension at or before 20 weeks, “late” hypertension
after 20 weeks, or no hypertension), alcohol use during pregnancy (using
the Alcohol use Disorders Identification Test (AUDIT-C) of any use
(AUDIT-C score>0) vs none and harmful use (AUDIT-C score≥3)
vs none and non-harmful),(17) any tobacco use during pregnancy, clinic
site where care was received, and enrollment period (Feb 2021-Jan 2022
vs Feb 2022-Dec 2022).