I have been studying many books to find and figure out what is happening inside your body each week you are pregnant. Each book was unique but I was able to collect from all of them to tell you what is most accurately happening in the womb each week of pregnancy.
The very beginning of your baby's life begins. The first week of pregnancy officially begins the date that you started your last period. Your body sheds its uterine lining to prepare to make a new one.
Your body has not ovulated yet but the starting of your period starts that whole cycle rolling. Technically, you are actually two weeks pregnant before you actually conceive.
The reason why doctors count the day of your last period as the starting of our first week of pregnancy is because it is hard to tell when you exactly got pregnant. But doctors know that it is somewhere between two weeks after the day of your last period and two weeks before the start of your next period. It just gives doctors a specific date to go by. It is easier to plan that way.
Also one thing that my sister taught me is how to tell when your due date is. She has studied the reproductive system in graduate school and is now a Physical Therapist. She said that if you take the day of your last period and subtract three months and then add a week that is your "tentative" due date. When I did it I was only one day off from the doctor. Pretty exciting!
At this point, your body does not react any differently.
Some books that I studied from are:
The Pregnancy Book - Everything you need to know from America's baby experts.
By: William Sears, M.D., and Martha Sears, R.N., and Linda Holt, M.D., F.A.C.O.G.
Your Pregnancy - Week by week
By: Dr. Glade Curtis, OB/GYN and Judith Schuler, M.S.
You & Your BABY Pregnancy - Your ultimate week-by-week pregnancy guide
By: Dr. Laura Riley, OB/GYN