Polyhydramnios; Vaccine exposure during pregnancy; This is a serious observational study case, initially received from other health professional on 16-Jul-2019, concerning a 19-year-old, adult female pregnant subject of body weight: 175 lbs, height: 67 inches and body mass index (BMI): 27.4, enrolled in, a  prospective observational safety study.  The subject had 2 previous pregnancy reported with 1 full-term live birth and 1 spontaneous abortion. There was no history of maternal or paternal congenital malformation. The subject's concomitant medications included Prenatal vitamins (minerals NOS, vitamins NOS) indicated for pregnancy.  The subject's last menstrual period (LMP) date was 03-Feb-2018 and the type of pregnancy was singleton. The subject had not used tobacco, alcohol or illicit drugs during pregnancy.   On 24-Apr-2018, the subject underwent haemoglobin (Hgb) electrophoresis and the result was normal as no major congenital malformation was noted. On 09-May-2018, the subject underwent an ultrasound and the result was normal as no major congenital malformation was noted. On 30-May-2018, the subject underwent an ultrasound, Quad screen and maternal serum alpha-fetoprotein (MSAFP)/serum markers test and result was normal as no major congenital malformation was noted.   The subject's current condition included anaemia (since 25-Jul-2018) which was treated with ferrous sulphate (from 28-Jul-2018 to 08-Aug-2018).   On 23-Aug-2018, the subject underwent an ultrasound and the result was normal as no major congenital malformation was noted.   On 11-Oct-2018, the subject underwent an ultrasound and the result was normal as no major congenital malformation was noted.   On 11-Oct-2018, at gestational age of approximately 35 weeks, the subject was administered Flu Vaccine Cell Subunit QIV NVD (influenza vaccine, subunit influenza virus vaccine polyvalent, batch number: 252664, dose, route of administration, anatomical location and expiry date: not reported) (explicitly coded as vaccine exposure during pregnancy) for an unknown indication.  On 22-Oct-2018 the subject underwent an ultrasound and the result was normal as no major congenital malformation was noted.   On an unspecified date, after vaccination, the subject experienced polyhydramnios.   On 06-Nov-2018, at gestational age of 39.4 weeks, the subject gave birth to a live birth to a normal female neonate via caesarean method of delivery. The neonate's birth weight: 3147 grams, head circumference: 34.9 cm and body length: 50.8 cm. The appearance, pulse, grimace, activity and respiration (APGAR) score at one minute was 7, at five minutes was 8 and no data was reported for APGAR score at ten minutes. The pregnancy complication was noted as polyhydramnios and no major congenital malformation identified.   On the same day, the subject had recovered from her current condition anaemia.  The outcome of the event polyhydramnios was not reported.   The estimated delivery date (EDD) was reported as 10-Nov-2018.; Reporter's Comments: A 19-year-old female pregnant subject experienced polyhydramnios, unspecified period after receipt of Flu Vaccine Cell Subunit QIV (considered as vaccine exposure during pregnancy). Due to lack of information regarding onset latency of the event polyhydramnios, the causal role of study vaccine assessed as unassessable. The event vaccine exposure during pregnancy assessed as not related considering the intended use of the vaccine. The case was assessed as serious (medically significant).