4. Singleton gestation ≥ 26 weeks 0 days gestation - ≤32 weeks 0 days gestation at the time of Visit 1 vaccination based on reconciliation of last menstrual period and ultrasound dating. Estimated due date (EDD) and Gestational Age (GA) - EDD will be based on reconciliation of a "sure" first day of the last menstrual period (LMP) and earliest dating ultrasound. If the LMP is uncertain, then the earliest dating ultrasound will be used to determine EDD and GA. If the ultrasound derived-EDD is in agreement with sure-LMP derived EDD, then the LMP-derived EDD is used to determine GA. If the ultrasound derived EDD is not in agreement with the LMP-derived EDD, the ultrasound-derived EDD is used to determine GA.