Why do we calculate the length of a pregnancy from the first day of a women’s last period?
In: 3216
If a woman is experiencing normal menstrual cycles, then her body exists in one of two states:
1) Preparing to be pregnant
2) Flushing the System
The system is flushed during her period. After that period ends, she is again preparing to be pregnant. The first step is preparing the uterus for the impending fertilized egg. The uterine lining will thicken. The fertilized egg showing up is just one step of many.
After a certain amount of time of the egg not showing up, this potential pregnancy is declared void and the woman then reenters the flushing the system state and the clock starts over.
Pregnancy is more than just the development that occurs to the zygote/embryo/fetus. The changes to the uterus that occur for pregnancy start after the last period ends. This will be at least a week before the introduction of sperm via intercourse or other means, and at least three weeks before the missed period that is the first signal most women get that they might be pregnant.
The best analogy I have is that you have a guest coming to stay, so you decide to build them a house for them to stay in. They may have come to visit you 2 weeks after you started building the house, and they may not have even let you know they were around for another 2 weeks, but when they leave the house you built them 36 weeks later, that house still stood for 40 weeks, not 36.
This is the “standard” cycle and “standard” pregnancy. There is a lot of variation.
Latest Answers