Habitual abortion is caused by complex and diverse factors, such as genetic factors, immune factors, endocrine factors, viruses, bacterial infections, and so on. Allogeneic antibodies, generated due to blood-group incompatibilities between a female and her fetus, are sometimes important for habitual abortion.A 26-year-old woman had undergone abortions 3 times in July 2015 (17 weeks pregnant), March 2017 (15 weeks of gestation) and February 2018 (16 weeks pregnant) before she came to the Reproductive Medicine Center of our hospital for prenatal examinations without pregnancy.Unexplained habitual abortion.A series of serological tests and nucleotide sequence of 1,4-galactosyltransferase (A4GALT) gene were performed.The patient was the rare p phenotype in P1P blood system and the patient's habitual abortion was caused by anti-PP1P antibody which was generated naturally in persons with p phenotype. There was a mutation (903C>G, CCC>CCG) in the 3rd exon of A4GALT gene, which is likely a significant contributor to p phenotype.This is the first case of habitual abortion caused by p phenotype due to independent 903C>G homozygous mutation with no similar record reported before, which indicates that it is a new class of mutation that leads to p phenotype.