Having easily produced a child is no guarantee someone can do it twice. In fact, around 10 per cent of the mums you may know have been struggling to have another child for more than a year. Some simply have trouble falling pregnant second time around, while others have trouble staying pregnant. And it's a cruel irony that women who want more babies already know the joy children bring, making each negative pregnancy test more crushing.
It's called secondary infertility, and it often goes unappreciated because the family already has one or more children.
Kirilee Furlong, 32, knows what it's like. After an easy conception and "perfect pregnancy" first time around, Kirilee was thrilled to have her daughter. But she knew her family wasn't complete.
Like many other women suffering from fertility problems, Kirilee experienced anger, sorrow, depression, self-blame and helplessness. She longed to give her daughter a sibling; for her, a brother or sister is not just a playmate, but a friend for life. "As they grow up, I want the kids to have a close family unit. Especially at Christmas time, as I'd like to have a lot of family around the table - I just think it's fun," she says.
Her road to another pregnancy included three miscarriages - "I pretty much fell pregnant every two months and lost them around seven to 10 weeks," she says - but the fourth pregnancy 'stuck'. Finally she had another daughter.
Kirilee was keen to relive each stage and rediscovered even more joys than she remembered. "It wasn't until I had my second daughter that I started to remember all those little moments that you had with your first baby," she says.
And the experience hasn't put her off trying for another baby - Kirilee still plans to have four children.
Not sure about what to say to a mum who may be experiencing secondary infertility? Kirilee shares her advice.
Don't say "Are you thinking of another one?"
Nosy comments add to the emotional burden. "After you have one child everyone says: 'You're due for another one!' 'Your daughter needs a brother or sister to play with.' That would often come at times when I was pregnant or had just had a miscarriage so I learnt from that never to say to anyone. Whether they have kids or not, just don't say 'when are you going to have babies?'"
Don't say "Be grateful with what you've got"
Some people think a couple should appreciate the child they've got instead of going to great lengths to conceive again, but longing for more babies doesn't mean they aren't grateful for their first. Such comments leave the couple feeling more isolated and misunderstood, and doesn't ease their heartache.
Don't say "We fell pregnant on the first try!"
If you're sharing pregnancy news of your own, try to tread carefully - there's no need to mention how easy conception was for you compared with the hard time they're having. Your friend is probably happy for you, but will be battling their own feelings, trying not to be jealous of those who haven't needed intrusive medical tests, or had to weigh the expense of IVF. Don't hide your pregnancy, but give them time to come to terms with it.
Don't say "You're lucky, I hate being pregnant"
Kirilee struggled with friends' complaints about their own pregnancies. "Around my second miscarriage I had two girlfriends who were pregnant at the same stage. They used to whinge about how sick they were and how tired they were and whether they could cope with this next baby. I used to think in my head: 'You could easily not have this baby.'"
Don't say "It will happen"
It's an easy thing to say, but try not to reassure friends that having a second baby will just take time if they've already been trying for a year. Unfortunately, no-one knows that's true. "As much as you hear it is going to happen for you one day, until it actually occurs, you don't have the reassurance that it is going to happen," Kirilee says.
How to help
Simply offer to be there if your friend wants to talk about it. Let them know that no matter what, you'll be there to help them get through it. "It does help to talk about it," Kirilee says.
If you want to do something practical, offer to babysit when your friends have medical appointments or just need a stress-free night out. You might wish for a magic wand, but at least listening will smooth your friend's journey towards their ideal family.