Gestational diabetes mellitus (GDM); Vaccine exposure during pregnancy; This is an observational study case, initially received from other health professional on 09-Dec-2019, concerning a 35-year-old, female pregnant subject of body weight: 185.8 lbs, height: 65 inches and body mass index (BMI): 30.9.  The subject had medical history of female infertility, she was exposed to Clomid (reported as Clomid pregnancy') and underwent intrauterine insemination. The subject's current condition included: advanced maternal age, obesity and high body mass index (BMP).  The subject's concomitant medication included Prenatal vitamins (unspecified vitamins and minerals) for vitamin and Tdap (diphtheria vaccine toxoid, pertussis vaccine acellular, tetanus vaccine toxoid) as vaccine.  The subject's obstetrical history noted that the subject had no previous pregnancies. The subject had no history of offspring with major congenital malformation (MCM). Also, the subject had no maternal or paternal history with MCM. The subject did not use tobacco, alcohol or illicit drugs during pregnancy.   The subject's last menstrual period (LMP) date was reported as 16-Jul-2019. The subject's estimated delivery date (EDD) and corrected estimated date of delivery (CEDD) were both reported as 21-Apr-2020. The type of pregnancy was singleton.  On 04-Sep-2019, the subject underwent ultrasound and no MCM was noted.  On 30-Sep-2019, at gestational age of 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: 259809, dose: 0.5 ml, route of administration, anatomical location and expiry date: not reported) (explicitly coded as Vaccine exposure during pregnancy') for influenza immunisation.  On 14-Oct-2019, the subject underwent first trimester screen and increased risk of Down syndrome was noted. No MCM was noted.  On 18-Oct-2019, the subject underwent MaterniT21 and no MCM was noted.  On 11-Nov-2019, the subject underwent second trimester screen and maternal serum alpha-fetoprotein (MSAFP/serum markers). No MCM was noted.  On 02-Dec-2019, the subject underwent anatomy ultrasound and no MCM was noted.  On 31-Jan-2020, the subject underwent glucose tolerance test and the result was abnormal.   On 10-Feb-2020, four months and 11 days after vaccination, the subject was diagnosed with gestational diabetes mellitus (GDM).  On 12-Mar-2020, the subject underwent growth scan (growth was 97%, as reported). No MCM was noted.  On 06-Apr-2020, six months and seven days after vaccination, macrosomia was noted in foetus. At gestational age of 39.1 weeks (as reported), the subject delivered a live birth, male neonate via caesarean method. The neonate's birth weight was 4621 grams, while head circumference and body length were not reported. Appearance, pulse, grimace, activity, and respiration (APGAR) scores were 8 (one minute) and 9 (five minutes). No data for APGAR score at 10 minutes was provided. No MCM was identified at birth.  At the time of this report, the subject had not recovered from the event of gestational diabetes mellitus.  The event of gestational diabetes mellitus was considered serious due to criterion of medical significance, per company.  The reporter did not provide the causality assessment.  This case is linked to the case 202003843 (the corresponding baby case).  Follow up information received from other health professional on 07-Jul-2020: Pregnancy outcome provided. Two labs added (glucose tolerance test and growth scan). Tdap added as concomitant medication. The event of gestational diabetes mellitus added, and the case was upgraded from non-serious to serious. Stop date added and outcome changed from 'not reported' to 'resolved'. In addition, baby case was opened, and link added. Narrative amended accordingly.  Company comment: A pregnant subject experienced gestational diabetes approximately three months after receiving Flucelvax QIV. The subject's current medical conditions included advanced maternal age, obesity and high body mass index. Therefore, causality was highly confounded. Causal role of the suspect vaccine is assessed as not related. Vaccine exposure during pregnancy assessed as not related per company's conventions.; Sender's Comments: A pregnant subject experienced gestational diabetes approximately three months after receiving Flucelvax QIV. The subject's current medical conditions included advanced maternal age, obesity and high body mass index. Therefore, causality was highly confounded. Causal role of the suspect vaccine is assessed as not related. Vaccine exposure during pregnancy assessed as not related per company's conventions.