This retrospective pregnancy case was reported in a literature article and described the occurrence of undescended testicle in a male infant. The mother received Hepatitis B vaccine. The infant was exposed to Hepatitis B vaccine during the first trimester of pregnancy. On an unknown gestation weeks, the infant was born via unknown delivery method. The mother's last menstrual period was unknown and estimated date of delivery was unknown. The subject's mother received Hepatitis B vaccine during the first trimester of pregnancy. The pregnancy resulted in a live neonate with a congenital anomaly. Unknown time after the mother's vaccination with Hepatitis B vaccine, the infant was diagnosed with undescended testicle. Serious criteria included congenital anomaly. Additional event(s) included fetal exposure during pregnancy, first trimester. The outcome of undescended testicle was unknown. The outcome(s) of the additional event(s) included fetal exposure during pregnancy, first trimester (unknown). The investigator considered that there was a reasonable possibility that the undescended testicle may have been caused by Hepatitis B vaccine. Additional information was provided. This retrospective pregnancy case was reported in a literature article and described the occurrence of undescended testicle in a male infant of unspecified age whose mother (aged between 14 and 41 years) was vaccinated with unspecified hepatitis B virus (HBV) vaccine (manufacturer unknown) during pregnancy (first trimester). The patient was part of the study, aimed to characterize the adverse events (AEs) after hepatitis B vaccination (approved for use in women of reproductive age) of pregnant women reported to the Vaccine Adverse Event Reporting System (VAERS), a spontaneous reporting surveillance system. The authors searched the VAERS for reports of pregnant women vaccinated in a specific country with hepatitis B vaccine from 01 January 1990 through to 30 June 2016. The authors included reports of women vaccinated with hepatitis B vaccine during pregnancy where the primary AE being reported (or the focus of the report) was in the live born infant. The authors excluded the reports where upon review they determined the subject was not pregnant or that hepatitis B vaccine was administered prior to the last menstrual period. All reports and available medical records were reviewed by physicians from the Food and Drug Administration and the Centers for Disease Control and Prevention to ascertain pregnancy status at time of vaccination, calculated the gestational age, and characterized the AEs. (In this study, VAERS received 192 reports involving hepatitis B vaccination of pregnant women. No maternal deaths were reported. Gestational age at the time of vaccination and at the time of the AE were calculated based on: clinical determination by the health care provider as stated in the report, earliest ultrasound assessment (if the former was not available), or last menstrual period, estimated delivery date, or estimated date of conception (if the first two options were unavailable) in the VAERS report and/or medical records. The authors used the following definition for trimesters: first (0-13 weeks), second (14-27 weeks), and third (greater than and equal to 8 weeks). The mother's last menstrual period was on an unknown date. Estimated date of delivery was on an unknown date. No information on patient's medical or family history or concurrent condition or concomitant medication was provided. On an unspecified date from 01 January 1990 through to 30 June 2016, the patient's mother received unspecified HBV vaccine (administration site and route unspecified; dosages unknown; batch number not provided) during pregnancy (first trimester) at 3-4 weeks gestation. The age of maternal vaccination was not provided. The patient was exposed to unspecified HBV vaccine in utero. [In this study, most (191; 99%) reports involved administration of a single antigen Hepatitis B vaccine. In 93 (48.4%) hepatitis B vaccine was given concurrently with other vaccines; 39 (43%) with known age were children and adolescents. The top three vaccines given concurrently with Hepatitis B vaccine were varicella, measles, mumps, rubella, and hepatitis A vaccines, respectively. Hepatitis B vaccines administered include: ENGERIX (93; 46.7%), RECOMBIVAX (85; 42.7%), no brand name (19; 9.6%) and COMVAX (1; 0.5%)]. On an unspecified date from 01 January 1990 through to 30 June 2016, an unknown period after the vaccination exposure in utero, the patient developed undescended testicle. This case has been considered serious due to congenital anomaly. (In this study, the definition of serious reports based on the Code of Regulations, was a report that includes one of the following: death, life-threatening illness, hospitalization or prolongation of hospitalization, or permanent disability. This definition was based on the reporter's assessment of the condition, and for pregnancy reports, it often is based on conditions in the mother and not necessarily the fetus or live born infant). The treatment was unknown. The outcome of the event was not reported. The authors stated "Strengths of VAERS include its broad national scope and ability to rapidly detect rare AEs and safety signals. However, as a spontaneous reporting system, it has important limitations, including reporting biases (over- or underreporting) and inconsistent quality and completeness of reports. Pregnant women who experience adverse outcomes in their fetuses or infants might be particularly susceptible to recall bias that influences decisions to report. Events that occur close to the time of vaccination and medically serious events are more likely to be reported. In addition, VAERS does not collect data on the number of individuals vaccinated: therefore, it is not possible to calculate the incidence or prevalence of AEs. Due to these limitations, we generally cannot determine from VAERS data alone whether a vaccine caused an AE". The authors concluded "Our analysis of VAERS reports involving hepatitis B vaccination during pregnancy did not identify any new or unexpected safety concerns. Additional data on the safety of maternal hepatitis B vaccination will be provided by an ongoing study of maternal and fetal adverse events after hepatitis B vaccine in the Vaccine Safety Datalink. Using data in VAERS, our safety review did not find any unusual or unexpected AE reporting patterns associated with hepatitis B vaccination in pregnancy". This is 1 of the 48 valid cases reported in the same literature article.