Kate Middleton appeared slimmer during her recent outing.

On Sunday, the Duchess of Cambridge stepped out with her 4-year-old son Prince George and 3-year-old daughter Princess Charlotte to support Prince William's charity polo match. Many noticed that Middleton has returned to her pre-baby body just a month after delivering Prince Louis.

"How does Kate Middleton basically lose her baby weight immediately after giving birth and I'll be six weeks into my summer fitness routine, still trying to lose my 'I ate too many snacks whilst revising' weight," one netizen asked.

According to Laura Spicer of The Women's Health Clinic, Middleton's post-baby body could be due to her genes and healthy pregnancy. The expert also explained that there are women who can easily get back into shape.

"Some women definitely can snap back into shape," Spicer said (via Express). "However, if you are fit and have had a healthy pregnancy with a normal BMI it is possible to be similar to your pre-pregnancy body shape. It's also got a lot to do with genetics, so if Mum recovered well you’re more likely to as well."

Spicer added that snacking on healthy food can also help lose weight. Meanwhile, NHS shared an advice to new moms to be fit and healthy after having a baby.

"Try to make eating well a priority. It will make you feel better, and healthy eating is important for the whole family. Aim to eat at least five portions of fruit and vegetables a day," NHS wrote. "Try cooking more than you need and freeze a couple of extra portions for another day. Tinned and frozen fruit and vegetables are quick to prepare, and they count towards your five portions a day."

Middleton is still on maternity leave, but she joined the royal family in celebrating Trooping the Colour on Saturday. The royal mom-of-three was even seen comforting Princess Charlotte after the latter accidentally slipped and hit her head on the railing.

According to royal biographer Duncan Larcombe, new royal moms usually take a six-month break from their official engagements. Thus, Middleton might not return to her duties soon. In fact, she "could disappear into the background until October."