There is no scientific evidence that during sexual intercourse there are some position that helps you get pregnant. Sexual position has no influence on the fertility or sex of the baby.
There are women who stay awhile lying down after sex to increase the chances of getting pregnant, but there is also no scientific proof of this.
However, there are studies that have shown that women undergoing artificial insemination and were lying for about 15 minutes, were 50% more likely to have a child than those who rose soon after.
So if a woman can lie down for a few minutes after intercourse, it may be that she favors fertilization and helps her become pregnant.
If you are trying to get pregnant and you can not, consult your family doctor or a gynecologist.