Continuation of burosumab during pregnancy in a patient with X-linked hypophosphatemia.

Burosumab is a well-established treatment for X-linked hypophosphatemia (XLH), but safety data during pregnancy are limited. We present the case of a woman in her 30s with XLH who continued to take burosumab during pregnancy to prevent symptomatic relapse. She gave birth to a male infant via cesarean section at 37 weeks without obstetric complications. The neonate exhibited a markedly elevated intact fibroblast growth factor 23 level {48 700 pg/mL (SI: 1933 pmol/L) (reference range, 19.9-52.9 pg/mL [SI: 0.79-2.10 pmol/L])} shortly after birth. This level declined rapidly to 151 pg/mL (SI: 4.7 pmol/L) by age 7 months but remained above the normal range. Genetic testing revealed a phosphate-regulating endopeptidase, X-linked gene mutation in both mother and child. The infant was initially managed with conventional therapy before transitioning to burosumab at 8 months of age. At the 1-year follow-up, the child demonstrated no apparent developmental delay or adverse skeletal events. This case presents real-world evidence of continued burosumab exposure during pregnancy in a woman with XLH. Although no apparent short-term maternal or fetal adverse events were observed, the safety of burosumab use during pregnancy is uncertain, and more clinical data, particularly with long-term follow-up, are needed.