Gestational diabetes mellitus; Vaccine exposure during pregnancy; This is an observational study case, initially received from other health professional on 23-Mar-2020, concerning a 30-year-old, pregnant, female subject of body weight: 152.6 lbs, height: 63.5 inches and body mass index (BMI): 26.6, enrolled in a prospective observational safety study.  The subject's medical history included increased BMI, hypothyroidism and human papillomavirus.  The subject's concomitant medications included Prenatal vitamins (unspecified vitamins and minerals) as vitamin, levothyroxine for hypothyroidism, Citranatal harmony (calcium citrate, colecalciferol, dl-alpha tocopheryl acetate, docosahexaenoic acid, docusate sodium, folic acid, iron pentacarbonyl, pyridoxine hydrochloride) as vitamin and Tdap (diphtheria vaccine toxoid, pertussis vaccine acellular, tetanus vaccine toxoid) as vaccine.   The subject's obstetrical history included one previous pregnancy with one spontaneous abortion. The subject had no history of offspring with major congenital malformation (MCM) and no relevant maternal and paternal medical history. The subject did not use tobacco, alcohol or illicit drugs during pregnancy.   The subject's last menstrual period (LMP) date was on 02-Jun-2019. The type of pregnancy was singleton. The subject's estimated delivery date (EDD) was reported as 08-Mar-2020 and corrected estimated date of delivery (CEDD) was reported as 17-Mar-2020.   On 06-Sep-2019, the subject underwent first trimester screen and nuchal translucency test and no MCM was noted.   On 03-Oct-2019, approximately at 17 weeks of gestation (calculated per LMP), the subject was administered Flucelvax QIV (Flu Vaccine Cell Subunit QIV NVD) [influenza vaccine, subunit influenza virus vaccine polyvalent, batch number: 261207, dose: 0.5 ml, route of administration, anatomical location and expiration date: not reported] for an influenza immunization (explicitly coded as 'Vaccine exposure during pregnancy').  On the same day, the subject underwent second trimester screen and no MCM was noted.  On 21-Oct-2019, the subject underwent anatomy ultrasound scan and no MCM was noted.   On 02-Jan-2020, two months and 30 days after vaccination, the subject developed gestational diabetes mellitus.   At gestational age of 39.9 weeks (as reported), the subject gave vaginally birth to live, female neonate of birth weight 3175 grams. The neonate's head circumference and body length were not reported. The appearance, pulse, grimace, activity, and respiration (APGAR) scores were 8 at one minute and 9 at five minutes. APGAR score for 10 minutes was not reported. No pregnancy complications were reported and no MCM was identified at birth.  At the time of this report, the outcome of event gestational diabetes mellitus was not resolved.  Per company, the event of gestational diabetes mellitus was considered serious due to the criterion of medical significance.   The reporter did not provide causality assessment to Flucelvax QIV.  Company comment: The subject experienced gestational diabetes a month after vaccination with the suspect product Flucelvax QIV. Chronology is plausible. Underlying medical history of increased BMI and hypothyroidism confound causality assessment. Nevertheless, causal role of vaccine is not excluded and is assessed as unlikely related (defaults to related for reporting purposes). Vaccine exposure during pregnancy was assessed as not related per company's conventions.; Sender's Comments: The subject experienced gestational diabetes a month after vaccination with the suspect product Flucelvax QIV. Chronology is plausible. Underlying medical history of increased BMI and hypothyroidism confound causality assessment. Nevertheless, causal role of vaccine is not excluded and is assessed as unlikely related (defaults to related for reporting purposes). Vaccine exposure during pregnancy was assessed as not related per company's conventions.