why only after 9 months parturation takes place it must take more 9 or less than that

Parturition is stimulated by FULLY DEVELOPED FOETUS AND PLACENTA. This stimulates the release of Oxytocin hormone whose function is to bring about uterine contractions.Average Gestation period generally lasts for about 280 days.
If the pregnancy is less than 38 weeks the baby born is a premature baby. If the pregnancy goes beyond 42 weeks, this is called a postterm pregnancy.