Why some ladies reach 40 w of pregnancy and pass their due date and still no normal labor?

Nobody knows why: Nobody knows what makes women go into labor. Something happens and there are changes the levels of certain hormones, such as decreasing Progesterone and increasing oxytocin, and then contractions start. Nobody knows why the pregnancy "clock" officially ends at 40 weeks (it's magic), but with some women delivering prematurely, and some going past 40 weeks.

Due date is estimate: Conception is calculated to occur (based on 28-day cycles), 2 weeks after the first day of the last menstrual period (lmp). Typical pregnancy lasts 10 lunar months or 40 weeks. Due dates are estimates based on a formula: lmp date+ 42 weeks=due date. For unknown reasons, first babies tend to be late but your doctor will induce labor before 42 weeks. Hope you have easy labor and a healthy baby!