"Having kids is something you have to think very carefully about when you're a full-time model, as it can often mean the end of your career," she explained.
"Some people can work right up until a few weeks before they give birth, but you pretty much have to stop working straight away.
"Ruth's so lucky in that she just pinged straight back into shape, she didn't even have to worry about her figure, but there are no guarantees," Sarah explained.
Ruth gave birth to her son just three months ago.
However, she was back on the catwalk five weeks after little AJ entered the world, modelling at an M&S fashion show.
At the time, Ruth said the secret to her super-quick weight loss is breast-feeding.
"I'm breast-feeding so that helps a lot, and he's so hungry as well so that's been brilliant," she said.
"I'd recommend it big time, you can actually feel your tummy contracting when you're breast-feeding and going back down to its normal size. I'm still not there yet, I'm a lot heavier than I was before, but sure no harm," she added.
Meanwhile, Ruth's close friend Sarah and fellow Assets model Aoife Cogan will be winging their way to Limerick next weekend for a catch-up with Ruth, as well as cooing over baby AJ.
But while Sarah is looking forward to doting over AJ, she revealed that she has no plans for starting a family of her own in the near future.
"Right now, I just want to keep modelling for as long as I can, I'm definitely not broody," she added.
However, she admits that when the time comes, she'll have had plenty of practice with children as she regularly gets roped in to babysit for her sibling's kids.
"I was baby-sitting for my nieces and nephews at the weekend and, while it was great to spend time with them, it was lovely to be able to hand them back at the end.
"We also did an Easter egg hunt on Sunday for them, and I just can't get over how much energy they have all the time.
"They're exhausting, I definitely wouldn't be ready for all that yet," she added.
The blonde beauty married her long-time love Kenny Vaughan at a ceremony in her local church of Glencullen last July and is hoping to plan something special to mark their first-year anniversary.
Sarah's birthday also falls around the same time of year, just one day before her wedding anniversary, so she is sure to be spoilt by her smitten hubby.