Hybrid

A human-vampire hybrid is a baby that is conceived when a human female and a vampire male mate. A hybrid can only be produced by a female human and a male vampire. Female vampires cannot reproduce as a female's body has to change to bear the child, and since vampires are frozen at the state of transformation, pregnancy is impossible.

Breaking Dawn
Breaking Dawn is the only book in the Twilight saga where hybrids make an appearance. When Bella and Edward Cullen went on their honeymoon, Bella fell pregnant. The pregnancy seemed fast-forwarded as Bella has only missed her period for 5 days, yet there was a small but noticeable bump around her hip. Bella decided to keep the child, as the hybrids do have a lovable presence for the mothers. She asked Rosalie Hale for help when Edward made it known that he was going to get the thing out of her. Rosalie, who has always wanted a child, agrees and guards Bella during the pregnancy, preventing an abortion. However, Bella's health was rapidly deteriorating due the strength of the hybrid, whom she called EJ (standing for Edward Jacob). The pregnancy also deprived her of her appetite and she could not eat food. From a snide comment that Jacob was thinking, Edward realized that the appetite of the baby may be closer to theirs than the humans. Carlisle and Rosalie agreed that Bella should drink blood. The blood worked, but the stronger Bella got, the stronger the hybrid got, too. By now it was breaking her bones and bruising her body. At further research, the Cullens discover that the hybrids usually rip their way out of the mother's body, killing the mother in the process. They decide on emergency vampirization for Bella. When Bella accidentally spilled a cup of blood on the couch, she reached for it too suddenly, triggering the birth. Rosalie, Jacob and Edward were working on getting the baby out, when Rosalie lost her concentration and gave way to her thirst. Jacob and Alice tow her away whilst Edward works furiously over Bella. The baby is born but Bella was falling under fast. Jacob refuses to hold the child and an in-control Rosalie comes back to take her, Renesmee, downstairs.

Three days later, Bella wakes up from the transformation, and sees Renesmee after hunting. Renesmee has grown at an incredible pace. She does not look like a three days old baby at all. Renesmee is very intelligent and still grows at a speedy rate. She prefers blood to human food, though she could survive on both.

From far away, Renesmee would look like an Immortal Child. She has a faint blush in her cheeks because there is blood pumping around her body and she also has a heartbeat, which sounds like a humming bird's wings fluttering.

Later, we are introduced to Nahuel another hybrid. He shares the same traits as Renesmee, but he stopped growing 7 and a half years after his birth. He is also venomous.

Nahuel tells of his father, Joham, who thinks he is creating a super race. Nahuel has three other half sisters.

Physical appearance
Human-vampire hybrids such as Renesmee Cullen, Nahuel, and his half-sisters briefly share traits with immortal children. They are incredibly beautiful and their skin seems as impenetrable as the immortal children hahahahahahahand normal vampires. So far it would seem that hybrid females have no venom, while Nahuel, the only known hybrid male, appears to have venom. Whether this is normal with hybrids or a rhahahahahahaandom chance, it is not known. The hybrids seem not to glitter in the sunlight, as vampires do, but their skin seems luminous.

Biology
It is currently unknown whether hybrids can reproduce. Though it is likely that they can as the body is changing until age 7 and a half where their appearance is comparable to a 16-year-old. They have 24 chromosome pairs, the same as shape-shifters, which lead discussion as to whether Renesmee can reproduce with Jacob. The hybrids are also a blind point in Alice's vision, another similarity with the shape-shifters. Their body temperature is unusually warm for vampires, just a few degrees below the norm for shape-shifters. This is mainly due to the blood in their systems. These hybrids can digest food, proving that their bodies do change, however possibly due to being exposed to it at birth, they prefer blood. Nessie, in particular, prefers human blood because Bella was drinking it for her to survive the pregnancy, while Pire, Nahuel's mother whom he killed at birth drank animal blood, giving Nahuel an early preference for animal blood. So far the only known hybrids are Nessie, Nahuel and Nahuel's 3 half-sisters.