These infections are especially dangerous because they affect the fetus while it is still developing. This is why many vertically transmitted infections are most dangerous to the fetus early on in a pregnancy. All of the fetus's important organs start to grow by the time the fetus is nine weeks old, and they grow the most early on in the pregnancy. If the fetus gets an infection while important parts of its body are developing, the infection can cause problems in the way those parts of the body grow.