What is the average number of stars for businesses in the Obstetricians & Gynecologists category?
Obstetricians & Gynecologists category refers to category_name = 'Obstetricians & Gynecologists'; calculation = AVG(stars)
SELECT CAST(SUM(T1.stars) AS REAL) / COUNT(T1.business_id) FROM Business AS T1 INNER JOIN Business_Categories AS T2 ON T1.business_id = T2.business_id INNER JOIN Categories AS T3 ON T2.category_id = T3.category_id WHERE T3.category_name = 'Obstetricians & Gynecologists'