This is a spontaneous case initially received from other health professional on 09-Apr-2019, concerning a 29-year-old, adult pregnant female subject of height: 70 inch, weight: 170 lbs, body mass index (BMI): 24.4, enrolled in a prospective observational safety study. The subject had 2 previous pregnancy reported with no birth defect noted. There was no history of offspring, maternal or paternal congenital malformation. The subject had a prior history of use of marijuana. The subject's historical conditions included allergic rhinitis (from 12-Jul-2018 to 21-Jul-2018), vaginitis (from 15-May-2018 to 23-May-2018), nausea (from 09-May-2018 to 06-Sep-2018), lower left quadrant/flank pain (from 15-May-2018 to 23-May-2018). The subject's historical drugs included: ZOFRAN and promethazine for nausea, METROGEL for vaginitis, FLONASE for allergic rhinitis. The subject's current condition included anemia (from 13-Sep-2018 to 12-Mar-2019). Concomitant medications included prenatal vitamins for pregnancy and ferrous sulphate for anemia. The last menstrual period (LMP) date of the subject was 25-Mar-2018. The type of pregnancy was singleton. On 09-May-2018, the subject underwent spinal muscular atrophy screening with no major congenital malformations noted. On the same day, the subject underwent haemoglobin electrophoresis test and no major congenital malformations were noted. On 09-May-2018, the subject underwent cystic fibrosis mutation analysis and no major congenital malformations were noted. On 17-May-2018, the subject had undergone ultrasound scanning and no major congenital malformations were noted. On 19-Jul-2018, the subject underwent maternal serum alpha-fetoprotein (MSAFP)/serum markers test which showed no major congenital malformations. On the same day, the subject had advanced trisomy screening with no major congenital malformations noted. On the same day, the subject had undergone ultrasound scanning and no major congenital malformations were noted. On 04-Sep-2018, the subject underwent renal ultrasound which showed no major congenital malformations. On 08-Oct-2018, at 28 weeks of gestational age, the subject was administered was administered Flu Vaccine Cell Subunit QIV NVD (batch number: 252664, dose, route of administration, anatomical location, expiry date: not reported) (explicitly coded as vaccine exposure during pregnancy) for an unknown indication. On 15-Oct-2018, the subject had undergone ultrasound scanning and no major congenital malformations were noted. On 19-Oct-2018, the subject underwent fetal fibronectin test which showed no major congenital malformations. On 19-Oct-2018, 11 days after vaccination, the subject had preterm labour. On 29-Oct-2018, 05-Nov-2018, 27-Nov-2018, 10-Dec-2018 and 17-Dec-2018, the subject had undergone ultrasound scanning and no major congenital malformations were noted. At 39.6 gestational weeks, the subject delivered a female neonate via vaginal method of birth weight 3027 grams, head circumference: 33 cm, length: 48.3 cm. The appearance, pulse, grimace, activity, and respiration (APGAR) score at one minute was 8 and at five minutes was 9. No pregnancy complications were noted. The expected date of delivery was reported as 30-Dec-2018. Reporter's Comments: A 29-year-old pregnant subject experienced preterm labor, 11 days after receipt of Flu Vaccine Cell Subunit QIV at 28 weeks of gestation (considered as vaccine exposure during pregnancy). Despite plausible temporal relationship, there is no evidence for a biological mechanism that could cause the event and maternal history multigravida (Gravida3), underlying medical condition anemia and use of marijuana during pregnancy provide more plausible explanation. Hence, the event preterm labor assessed as not related to the study vaccine. Subject's historical and concomitant medications represent a confounding factor. The event vaccine exposure during pregnancy assessed as not related considering the intended use of the vaccine. The company assessed the case as serious (medically significant).