As the years roll by and sleepless nights with a newborn become a distant memory, you almost have that pre-baby bounce back in your step. But for some mums, there’s something creeping up behind them: baby fever.

“When I was 39 I woke up one day and thought, ‘Wait! I'm not finished!’” says Nicole McLachlan. “The feeling came out of nowhere but it was intense and I knew without a shadow of a doubt that I had to have another child.”

This was an about-face for the mum who had declared she was “100 per cent certain” she wouldn’t have another baby after she’d welcomed a son five years earlier. Severe postnatal depression followed by challenging toddler years had given McLachlan every reason to stop at one child – and with two older stepchildren, she also felt another child would be too much for her husband to cope with.

But when baby fever struck, she discovered her husband was actually “very agreeable” to the idea, and a baby girl was soon on the way.

This extreme emotional desire for a baby, often going against long-held beliefs or rational thought, is what defines ‘baby fever’. A recent study found that baby fever, also described as feeling clucky or broody, isn’t just something portrayed in popular culture, but is a “distinct psychological phenomenon”.

One of the study’s researchers, Dr Gary Brase, says of the findings: “What we found was that some of the key things that led to baby fever (or, conversely, an aversion to having babies) were seeing and hearing baby-related things.”

Seeing friends with their babies was something Vicki Williams says triggered her desire to expand the family she had considered complete for many years. Having married relatively young, she’d had a son when she was 24, then a daughter a few years later. She says she felt content and blessed with her “pigeon pair”, but when her daughter started school and many of Williams’ friends began having their own babies, her feelings started to change.

“Later, when my best friend announced she was pregnant with her third baby, I really, really felt that I wanted another baby more than ever,” she remembers.

However, with a history of endometriosis, Williams had difficulty falling pregnant. So after several years of trying for a baby and with her children aged nine and 11, Williams and her husband made an appointment at an IVF clinic. She says, “Walking into that clinic made me want another baby more than ever.”

She became pregnant after a second attempt at IVF, at the age of 35. “I wasn’t as fit as I was in my younger days, so being pregnant again was a real shock,” she says.

WIlliams also had her other children to consider. Williams’ son was excited about the baby news but her daughter’s reaction was different: “She cried for over an hour. She felt I was going to love her less at that time and that the baby was going to replace her.”

Williams says she made an effort to include her older children in the pregnancy and birth. “They attended the ultrasounds and were outside waiting when the baby was born. They even got to have the first cuddle.” Now more like a “second mum” to her much younger brother, Williams’ daughter quickly fell in love. “Seeing my older children with our new baby and showing him so much love was such a lovely experience … I felt really happy, our family was complete.”

Clinical psychologist Lynn Jenkins agrees that efforts to include older children may help ease the transition when expecting a baby. Painting a realistic picture of life with a newborn and reassuring older children that they have a firm place in the family and a special place in your heart are also things that may help, she says.

Parental guilt and challenging behaviours from existing children are some of the challenges that may come up, she says.  She cautions that the decision to add a baby to an established family unit deserves careful thought.

"It’s probably wise to sleep on it for a while and to ask yourself, ‘What will I be risking by adding a new baby to my existing family?’ versus, ‘What will I be risking by not adding a new baby?’” she advises.

And sometimes there are unexpected challenges. Just nine months after her longed-for third child arrived, Williams was shocked to discover she had fallen pregnant again. “It was like having twins. We had to buy a bigger house and a larger car. It’s been really hard work … but we couldn’t imagine life without both of them now.”

McLachlan says that the day-to-day juggle is tricky but is not the hardest part for her: “It’s the difference in life stages and interests a bit further down the track that cause an impact on the whole family.

“But it’s worth every tricky, frustrating moment.”