Periodontal disease (PD) in pregnancy is associated with an increased risk of adverse pregnancy outcomes including miscarriage and preterm birth. Evidence exists that periodontal disease treatment may reduce inflammatory mediators in gingival crevicular fluid (GCF) and the risk of inflammation-associated pregnancy complications. The aim was to determine if periodontal disease treatment during mid-pregnancy alters local inflammation in GCF and has beneficial effects on clinical dental parameters. Eighty pregnant women with clinically diagnosed PD were recruited from a randomised controlled trial on the treatment of periodontal disease in pregnancy conducted in Perth, Australia. The treatment group underwent intensive PD treatment (20–28 weeks’ GA), while the control group underwent the same treatment postnatally. GCF was collected at 20 and 28 weeks’ gestation and concentrations of cytokines determined by multiplex assay: IL-1β, IL-6, IL-8, IL-10, IL-12p70, IL-17, TNF-α and MCP-1. Periodontal treatment significantly reduced the GCF levels of IL-1β, IL-10, IL-12p70 and IL-6 at 28 weeks’ GA compared with controls, while levels of MCP-1, IL-8 and TNF-α exhibited a significant gestational age-dependent increase, but no treatment response. Post-treatment clinical parameters improved with significant reductions in bleeding on probing, clinical attachment loss, and probing depth. No changes in pregnancy-related outcomes were observed, although the severity of periodontal disease was significantly associated with an increased risk of infants born small for gestational age. PD treatment in pregnancy reduces the levels of some inflammatory mediators in the GCF and improves dental parameters, with no overt effects on pregnancy outcome.