Gestational diabetes mellitus; Vaccine exposure during pregnancy; This is an observational study case, initially received from other health professional on 25-Mar-2020, concerning a 30-year-old, pregnant female subject of body weight 165 lbs, height 61.2 inches and body mass index (BMI) 31, enrolled in, a prospective observational safety study.  The subject's medical history included obesity, with body mass index 31, and seizure disorder.  The subject's concomitant medications included Vitamins (unspecified vitamins) for OB supplementation.   The subject's obstetrical history noted that the subject had no previous pregnancies. The subject's maternal history included autism in subject's nephew and no relevant paternal medical history with MCM. The subject did not use tobacco, alcohol or illicit drugs during pregnancy.   The subject's last menstrual period (LMP) date was 29-Sep-2019. The subject's estimated delivery date (EDD) was reported as 05-Jul-2020, while the corrected estimated date of delivery (CEDD) was reported as 12-Jul-2020. The type of pregnancy was singleton.  On 27-Nov-2019, the subject underwent initial ultrasound and no MCM was noted.   On 03-Jan-2020, at gestational age of approximately 11 weeks (calculated per LMP), the subject was administered Flucelvax QIV (Flu Vaccine Cell Subunit QIV NVD) [influenza vaccine, subunit influenza virus vaccine polyvalent, batch number: 261223, dose: 0.5 ml, route of administration, anatomical location and expiration date: not reported] (explicitly coded as Vaccine exposure during pregnancy') for an influenza immunization.  On the same day, the subject underwent first trimester screen and nuchal translucency scan. No MCM was noted.   On 31-Jan-2020, the subject underwent second trimester screen and maternal serum alpha fetoprotein (MSAFP) / serum markers screening. No MCM was noted.  On 04-Feb-2020, approximately one month after vaccination, the subject developed gestational diabetes mellitus.   On 09-Feb-2020, the subject started treatment with metformin at a dose of 500 mg, administered two times per day via oral route.   On 28-Feb-2020, the subject underwent anatomy ultrasound and no MCM was noted.  At the time of initial reporting, the subject had not recovered from the event.  The event of gestational diabetes mellitus' was assessed as serious due to the criteria of medical significance, per company.  The reporter did not provide causality assessment.  Company comment: A 30-year-old pregnant female subject developed gestational diabetes after vaccination with the suspect product Flucelvax QIV for an influenza immunization. Reportedly, approximately one month after vaccination the event has occurred. Chronology seems plausible. The subject's medical history included obesity, with body mass index 31, and seizure disorder. The subject's concomitant medications included Vitamins (unspecified vitamins) for OB supplementation. With information presented, causality is assessed as unlikely related considering medical history of the patient. Exposure during pregnancy is assessed as unrelated per company conventions. (''unlikely related'' defaults to "related" in the safety database for reporting purposes); Sender's Comments: A 30-year-old pregnant female subject developed gestational diabetes after vaccination with the suspect product Flucelvax QIV for an influenza immunization. Reportedly, approximately one month after vaccination the event has occurred. Chronology seems plausible. The subject's medical history included obesity, with body mass index 31, and seizure disorder. The subject's concomitant medications included Vitamins (unspecified vitamins) for OB supplementation. With information presented, causality is assessed as unlikely related considering medical history of the patient. Exposure during pregnancy is assessed as unrelated per company conventions. (''unlikely related'' defaults to "related" in the safety database for reporting purposes)