In the medical world, when the pregnancy time exceeds the actual time, it is called prolonged or post term pregnancy. Basically, normal pregnancy or at term occurs for 38-41 weeks. Besides, the pregnancy time which exceeds more than 42 weeks is categorized as post term. The number of pregnant mother who has pregnancy time more than limit time is relatively small; it is only about 3 to 12%.
The number of pregnancy time which exceeds the normal limit is not that much. Usually, these cases happens because of there is miscounting when calculating the pregnancy age. This miscounting can be happened because of the menstruation cycle of the mother is irregular. It means, miscounting of the pregnancy time happens from the beginning when the mother knows that she is pregnant.
To know the exact time when the mother starts to be pregnant, it is good if the mother does the USG checking. By doing checking through USG and measuring the height of uterus regularly in the early trimester of pregnancy, pregnant mother will know the exact time of fertilization in the uterus happens. It happens because when she gets late menstruation it is not sure that the fertilization has occurred in her uterus. It is possible if the fertilization happens about 2-3 weeks after the mother gets late menstruation.
However in some cases, even though the mother has done several checking like what is mentioned above, there is always childbirth which is missed from the estimation day. And until today, there is no clear way the cause of the missed of this birthing time. Some obstetricians guess that the miscount of the birthing time can happen because of mother factor and also the embryo in the uterus.
If it is seen from the mother factor, post term pregnancy can happen because of the genetic and heredity factor from the previous pregnancy. Besides, post term pregnancy can also happen because of genetic or heredity factor, the lack of sulfates enzyme in the placenta, the lack of thyroid hormone number, and many others. On the other hand, when it is seen from the embryo factor which is in the uterus, post term pregnancy can happen because of there is mistake in the embryo place or the growing abnormality in the skull. Or, this kind of pregnancy can happen because of the embryo is in the big size or often called microsomal. Some people also believe that post term pregnancy often occurs to the male embryo instead of female embryo; even though there is no research which says that this statement is true.