Obviously the birth control didn’t work this time as Eloise, a 37-year-old siamang, had her seventh baby this month.

Eloise has been on chemical contraception for years, after she had six babies over her life, but something didn't work as intended, the San Diego Tribune reported.

“We’re not certain why birth control didn’t work in this case, but as with humans, it is not uncommon for contraceptive failure to happen from time to time,” Jill Andrews, animal care manager at the San Diego Zoo, said.

>> Read more trending news

Eloise, according to a statement released by Andrews, didn’t show signs that she was pregnant.

It's the first baby siamang in 12 years, KUSI reported.

All types of gibbons, including siamangs, are endangered, due to loss of habitat from logging and agriculture. They are also sold when they're young as pets. Mothers are usually killed to get to their offspring for the illegal trade, the San Diego Tribune reported.

Siamangs mate for life and share responsibility for raising their young. They can’t be kept apart since they are social animals, and being together helps them thrive in captivity.

The zoo is not sure if the new baby is a boy or a girl but officials plan on finding out over the next few months. Until then, they will allow mom and baby to stay together, the San Diego Tribune reported.