It might seem strange, but you are not pregnant the first week or two of the time allotted to your pregnancy. Conception typically occurs two weeks after your last period. This means your period is counted as part of your pregnancy, even though you are not pregnant. The first week or two is 7-14 days, add that to the 18 days, you get 25-32 days. 25-32 days is 4-5 weeks. Some of the sites I get information from are women talking to each other about their pregnancy. On average most women hear the babies heart at 4-5 weeks, some heard it earlier and others later. Other information, I got from the Mayo Clinic and my doctor. As for the doctor, it takes one full of dog doo doo to know another full of dog doo doo.