A Study of Nipocalimab or Intravenous Immunoglobulin (IVIG) in Pregnancies At Risk of Fetal and Neonatal Alloimmune Thrombocytopenia (FNAIT)

Inclusion Criteria:

* Pregnant and an estimated gestational age (GA) from week 13\* to 18 at visit 1

  \*Randomization for high-risk FNAIT participants to occur at GA Week 12
* Has a history of greater than or equal to (\>=) 1 prior pregnancy with FNAIT based on medical records including: a) neonatal platelet count less than (\<) 150\*10\^9/Liter with no fetal/neonatal intracranial hemorrhage (ICH) or severe fetal/neonatal hemorrhage (standard-risk) OR b) fetus/neonate with ICH or severe hemorrhage in a fetus/neonate (high-risk)
* Current pregnancy with presence of maternal anti-HPA-1a and/or anti-HPA-5b alloantibody and positive fetal HPA-1a and/or HPA-5b genotype as confirmed by cell-free fetal DNA in maternal blood
* Health status considered stable by the investigator based on physical examination, medical history, vital signs, 12-lead electrocardiogram (ECG), and clinical laboratory tests performed at screening
* For maternal participant and neonate/infant, willing to forego participation in another clinical study of an investigational therapy until the last follow-up visit

Exclusion Criteria:

* Currently pregnant with multiple gestations (twins or more)
* History of severe preeclampsia in a previous pregnancy
* History of myocardial infarction, unstable ischemic heart disease, or stroke
* Known allergies, hypersensitivity, or intolerance to nipocalimab or its excipients, to IVIG or to prednisone
* Has any confirmed or suspected clinical immunodeficiency syndrome or has a family history of congenital or hereditary immunodeficiency unless confirmed absent in the participant