We meet Pairi Daiza's twin baby pandas ahead of their unveiling next month

There is a collective "ahhh!" from the visitors at Pairi Daiza's panda enclosure when a woman walks out of a side door with a three-month old cub in her arms. The panda, who currently goes by the name Baby Boy, weighs 6kg and is the size of a watermelon. He is sleepy, and when placed in his cushioned basket, immediately rolls into a sleep position, next to his twin sister, provisionally named Baby Girl.

The panda cubs are the pride and joy of Pairi Daiza, near Mons, that boasts some of the most lavish and extensive animal collections of any zoo in the world. Over 7,000 animals from 700 different species are scattered around its 80-hectare estate, which attracts around two million visitors a year.

But it is the panda family that is the biggest draw. Baby Boy and Baby Girl were born on 8 August - their final names will be revealed on 16 November after an online vote - and they have been kept in incubators and a closely sealed clinic until now. Last week, The Bulletin visited them, as a prelude to their official unveiling to the public on 14 December.

When they were born, they were described by zoo staff as "pink naked sausages" and "shrimps", but now they are fuzzy, clumsy fur balls. They are the children of mother Hao Hao and father Xing Hui, 10, and weighed just 160g (Baby Girl) and 150g (Baby Boy) when they were born. They also have a big brother, Tiao Bao, or "treasure of heaven", who was born at the zoo in June 2016.

Giant pandas are notoriously tricky reproducers: females are in heat only during a one-to-three-day window a year, while the males often lack any bedroom savoir faire. “So we did the mating for them,” says Tim Bouts, Pairi Daiza’s zoological director.

They had to scramble in April as they tried to pinpoint the moment of peak estrogen. “We follow every pee that she does, every two or three hours, just to make sure we have the peak. Then we basically have 24 hours to do the artificial insemination,” he says.

Timing is everything. There were 13-14 veterinarians from China and Belgium specialised in fields such as ovary ultrasounds and male reproductive tracts. They first took Xing Hui’s sperm, which had to be fresh for the two inseminations. Both pandas were fully anesthetised.

After the procedure, they had a long wait to see if the conception would lead to a real pregnancy or a "false" pregnancy that pandas sometimes exhibit. “There is a delayed implantation,” Bouts says. “After the artificial insemination, the embryo will divide and float in the uterus, but we don’t speak of a pregnancy, just conception. From the point of artificial insemination until the implantation, we don’t even know if she’s pregnant.”

It was only about three weeks before birth that they knew for sure that Hao Hao was pregnant. “And we were over the moon when we found out it was twins,” Bouts says.

Hao Hao and Xing Hui are on a 15-year loan from China and arrived in 2014. China still sees its pandas as national treasures and, with only around 2,000 left in the wild, is sparing about renting them out: Pairi Daiza is one of just 27 zoos in 21 countries (10 in Europe) with giant pandas.

Although China once gave the bears away as ambassadors, the era of panda diplomacy has been replaced by a shrewd renting arrangement – Pairi Daiza paid an estimated €10 million to host the duo in 2014. That doesn’t include the costs of facilities and care for the animals – including a specially built enclosure, and Liu Yang, who has been their Chinese "panda handler" since 2014 - or the extra €500,000 or so for every cub that is born. That will be more than repaid over their stay, with visitor numbers jumping sharply after they joined.

Pairi Daiza is keen to stress that it is more than just the panda zoo. It is involved in saving and breeding 89 endangered species. For example, the zoo has 22 elephants and one of their research projects involves treating elephant herpes and donating blood to help infected elephants in other parks. But for the moment, its adorable panda family is all that visitors - and indeed staff - are talking about.