Gestational diabetes mellitus; Vaccine exposure during pregnancy; This is an observational study case, initially received from other health professional on 26-Aug-2019, concerning a 35-year-old, pregnant, female subject of body weight 222 lbs, height 63.5 inches and body mass index (BMI) 38.7, enrolled in a prospective observational safety study.  The subject's obstetrical history noted that the subject had no previous pregnancies. The subject had no history of offspring with major congenital malformation. Also, the subject had no maternal or paternal history with major congenital malformations. The subject did not have a prior history of use of tobacco, alcohol or illicit drugs during pregnancy. The subject's concomitant medications included Prenatal Vitamins (mineral NOS, vitamins NOS).   The subject's last menstrual period (LMP) date was on 12-May-2018 and estimated delivery date (EDD) was 16-Feb-2019. The type of pregnancy was singleton.   On 15-Jun-2018, the subject underwent cystic fibrosis mutation analysis and spinal muscular atrophy and no major congenital malformation was noted.  On 29-Jun-2018, 11-Jul-2018 and 12-Sep-2018 the subject underwent ultrasound scan and no major congenital malformation was noted.  On 10-Oct-2018, at gestation age of 21 weeks, the subject was administered Flucelvax QIVc [influenza vaccine, subunit influenza virus vaccine polyvalent, batch number: 252664, dose: 0.5 ml, expiration date, route of administration and anatomical location: not reported] (explicitly coded as 'vaccine exposure during pregnancy') for immunization for influenza virus.  On 14-Nov-2018, approximately one month after vaccination, the subject developed diabetes mellitus.   On 28-Nov-2018 and 03-Jan-2019, the subject underwent ultrasound scan and no major congenital malformation was noted.  On 14-Jan-2019, 22-Jan-2019, 25-Jan-2019, 29-Jan-2019, 30-Jan-2019, 31-Jan-2019, 04-Feb-2019 and 07-Feb-2019, the subject underwent non-stress test and no major congenital malformation was noted.  At gestational age of 38.9 weeks, the subject delivered live birth, male neonate by caesarean method. The neonate's birth was 3033 g, head circumference was 35.6 and length was 52.7. The appearance, pulse, grimace, activity and respiration (APGAR) scores were 8 (one minute), 9 (five minutes) and no data was reported for APGAR score at ten minutes. No major congenital malformation was identified at birth.  At the time of initial reporting, the subject had not recovered from the event of 'diabetes mellitus'.  Company comment: A 35-year-old, pregnant, female subject experienced Gestational diabetes mellitus one month after exposure to the suspect product - Flu Vaccine Cell Subunit QIV NVD. Chronology seems plausible. The patient's  body weight 222 lbs ( body mass index (BMI) 38.7) provides more plausible aetiology for developing Gestational diabetes mellitus. With information provided, causality is assessed as not related to the product.; Reporter's Comments: A 35-year-old, pregnant, female subject experienced Gestational diabetes mellitus one month after exposure to the suspect product - Flu Vaccine Cell Subunit QIV NVD. Chronology seems plausible. The patient's  body weight 222 lbs ( body mass index (BMI) 38.7) provides more plausible aetiology for developing Gestational diabetes mellitus. With information provided, causality is assessed as not related to the product.