When a mommy and daddy love each other very much, the daddy's sperm meets the mommy's egg inside the mommy's tummy. This is called fertilization and it makes a tiny baby.
Some people believe that the baby starts to grow and develop right away after fertilization, and every day counts as the start of the pregnancy. But other people believe that it takes a little while for the fertilized egg to attach to the mommy's uterus and start growing, and that's when the pregnancy really begins.
This is a big debate because it affects things like when a woman can get an abortion, or where certain medicines can be used to help a woman's pregnancy. Some people believe that the fertilized egg is a tiny human, and that it's always wrong to end a pregnancy. Other people believe that the mom's health and well-being should be the most important thing to consider, and that she should have the right to make choices about her own body.
It's a complicated topic, but the most important thing is that everyone should be respectful of each other's opinions and make sure that pregnant women are getting the best care possible.