State the average period of Ms. Angelena Kertzmann's several normal pregnancies.
DIVIDE(SUBTRACT(stop time - start time), COUNT(DESCRIPTION = 'Normal pregnancy')));
SELECT CAST(SUM(strftime('%J', T2.STOP) - strftime('%J', T2.START)) AS REAL) / COUNT(T2.PATIENT) FROM patients AS T1 INNER JOIN conditions AS T2 ON T1.patient = T2.PATIENT WHERE T1.prefix = 'Ms.' AND T1.first = 'Angelena' AND T1.last = 'Kertzmann' AND T2.description = 'Normal pregnancy'