Yes, it is possible.
Teratozoospermia is an increase in the percentage of abnormal spermatozoa. Nowadays, any sperm that is not perfect in its shape is considered abnormal. Many spermatozoa fall into this category, which is why most males do not have a percentage of normal spermatozoa of more than 10%. When the proportion is excessive (greater than 96%), the patient is considered to have teratozoospermia.
In general, sperm need to be normoconformed to perform their function correctly. However, it is absolutely normal to have sperm that are not normal in shape. In fact, in every male there are many more abnormal sperm than normal sperm, so that the abnormal sperm do not prevent healthy sperm from functioning.
Abnormal sperm will have a much lower chance of fertilising an oocyte, so their percentage increase in the sample will decrease the probability of pregnancy. However, teratozoospermia is a percentage concept, there will always be normal sperm that can fertilise the oocyte (and even some abnormal sperm, if the problem is mild). This is why a male with teratozoospermia can achieve a natural pregnancy... although this problem is not usually pure, but is often associated with a general decrease in sperm quality (sperm count/motility).
