Background A prior population based study found the odds of low birth weight for women with inadequate weight and history of Chlamydia trachomatis (Ct) infection during the pregnancy was nearly twice that of women without Ct infection (AOR 1.98, p < 0.02). A stronger association was observed with pre-term low birth weight (AOR 2.34, p < 0.01). Subsequently, 2005–2006 ratified state laws and regulations required Ct screening during pregnancy.

Objectives To examine associations between Ct infections during pregnancy and birth outcomes among women who gave birth in Florida after implementation of mandated Ct testing.

Methods Logistic Regression analysis of Florida birth records (548,407) linked to records for women who tested positive for Ct (12,334), were treated and gave birth from 01/01/2008 to 12/31/2010.

Results Among 18–35 years old, with singleton births, 2.7% were reported with chlamydia during pregnancy and 6.5% experienced low birth weight (LBW) outcome, less than 2500 grammes. After controlling for smoking, prenatal care, race, education, marital status, inadequate weight gain and nine other covariates, Ct infection treatment in gestation weeks 17 through 28 was associated with increased odds of delivering a moderately LBW infant (1500 to 2499 grammes) (AOR 1.29, 95% CI 1.12 to 1.49). No association was found for very LBW infants (< 1500 grammes) or for Ct infections treated before 17 weeks gestation. The increased odds for moderately LBW were observed for preterm (< 37 weeks gestation) moderately LBW infants (AOR 1.28, 95% CI 1.07 to 1.53) and term (≥ 37 weeks gestation) moderately LBW infants (AOR 1.27, 95% CI 1.03 to 1.57).

Conclusions Proactive public health policies for chlamydia screening during pregnancy and intensified treatment timeliness may have contributed to improved pregnancy outcomes, but Ct infection continues to be associated with increased odds of moderately LBW. Prospective examination of treatment dosage and timeliness may prove informative.