You see, the zoo keepers had no idea that the critically endangered primate was even on the horizon, because they'd done several pregnancy tests on the baby's mother that had all returned negative results.
That meant that the wildlife experts had no idea that Leia the orangutan was pregnant until her new arrival unexpectedly - well - arrived.
This is fantastic news, for a lot of reasons, but mostly because the Bornean orangutan is one of the rarest animals in the world, and one of those at highest risk of extinction, too.
According to the experts, this is because of the destruction of their habitat, as well as the conversion of their home into unsustainable palm oil plantations, and illegal hunting.
As such, estimates suggest that there could be as few as 55,000 Bornean orangutans remaining in the wild on their island homeland, which is the only place that they are found.
Which makes this occasion even more worthy of celebration.
Chester Zoo primate keeper Chris Yarwood said: "The pregnancy tests we had carried out on Leia in the months prior to the birth had actually returned negative results. It was therefore a wonderful surprise to arrive one morning to see her protectively cradling a beautiful new arrival."
"Leia enjoys spending lots of time alone with her baby and has so far been quite shy about showing it off.
"She always keeps it really close to her and so we've not yet been able to clearly determine what the gender of the infant is.
"What we are sure of though is that the baby is bright, alert and suckling well from mum and has developed well over the last couple of months.
"This is Leia's second baby - she's a great mum and is doing a fab job once again.
"Chester is one of the few zoos in Europe that cares for both Bornean and Sumatran orangutans. These are critically endangered animals and, importantly, we've seen babies from both sub-species born in recent times.
He added: "It just goes to show that, despite all of the uncertainty in the world right now, life is carrying on as normal for the orangutans, which is really uplifting to see."
Dr Nick Davis, the zoo's Deputy Curator of Mammals, said: "Bornean orangutans are the largest arboreal mammals in the world and how fast their numbers are plummeting is frightening.
"They are victims of illegal hunting and habitat loss and are highly threatened by the unsustainable oil palm industry, which is having a devastating effect on the forests where they live.
"These magnificent animals are being pushed to the very edge of existence and it really could be the case that we soon lose them forever.
"It's absolutely vital therefore that there's a sustainable population of Bornean orangutans in the world's progressive zoos - every addition to the European endangered species breeding programme is so, so important."
"There's still a huge need to tackle the excessive deforestation in Borneo and show people everywhere that they can make a difference to the long-term survival of orangutans.
"We really hope that Leia's new baby helps to further highlight how simple everyday choices, like choosing products which contain only sustainably sourced palm oil, can have a massive impact on the future of these remarkable animals."