産婦人科医が減っているのは出生数の減少で医療ニーズが低減したのを反映している。

。

The decline in obstetricians reflects the reduced demand for medical care caused by the drop in the birth rate.

Literal

Obstetrician-gynecologists [subject-が] are-decreasing [nominalizer-の] [topic-は] birth-number [attributive-の] decrease [by-で] medical-needs [subject-が] have-decreased [nominalizer-の] [object-を] is-reflecting-[continuous-ている].

Editorial/medical prose. 産婦人科医 ('obstetrician-gynecologist') is a compound 産婦人科 ('obstetrics-gynecology' department) + 医 ('doctor'). 反映する ('to reflect, to mirror') is the standard verb for causal/statistical reflection. Note the double nominalization: two ~のが clauses act as subject and object of 反映している.