How many patients with an Ig G lower than normal has the symptom of abortion?
Ig G lower than normal refers to IGG < 900; symptom of abortion refers to Symptoms = 'abortion';
SELECT COUNT(DISTINCT T1.ID) FROM Patient AS T1 INNER JOIN Laboratory AS T2 ON T1.ID = T2.ID INNER JOIN Examination AS T3 ON T3.ID = T2.ID WHERE T2.IGG < 900 AND T3.Symptoms = 'abortion'