It is a common occurrence in hospitals to send a placenta down to the lab for testing if an abnormality is suspected. If this happens in your case, the placenta will not be able to be encapsulated.
I have observed, however, that sending the placenta to pathology occurs much more frequently than actually necessary. There are differences amongst doctors as to what they feel would constitute a necessity of sending the placenta, but there are some doctors who send nearly every placenta for the tiniest of abnormalities. There are also many doctors who will send the placenta when a birth is particularly difficult, which often has much more to do with the protocols and interventions of the hospital than something to do with the placenta.
Having spoken with laboratory technicians, most often when a placenta is sent to pathology, no significant information is obtained.
Also, it is important for women to be aware that hospitals can keep and sell placentas without notifying the women to whom the placenta belongs. There is a lot of money to be made in this area. Placentas are also routinely kept for research experiments. They do not have to obtain your consent to do this.
If your physician wants to send your placenta instead of let you take it home, my recommendation would be to ask questions. Why does your physician want to send it? What abnormalities is he/she looking for? What information is hoped to be obtained? If there a benefit to you or the baby? If you had a difficult birth, does your doctor believe this will help prevent the occurrence next time? Often times, I have found that a doctor is simply being overly cautious and doesn't have a sufficient answer to these questions.
These are good things to discuss with your physician beforehand as well. Express your adamant desire to take your placenta home and find a physician who will only send your placenta to the laboratory under certain circumstances agreed upon with you in advance in which case your physician has a significant reason why it would be necessary.