Is there an ideal age gap for siblings?

Four months after giving birth to her first baby, Emily Carter was expecting again.

"Falling pregnant with Liam so soon after Liv was born wasn't a planned decision [and] I was absolutely worried about them being so close in age and what that would mean for them both," the 38-year-old from Canberra says.

"My main concerns were that I wouldn't have enough time to give them the one-on-one attention that they deserved."

Like Emily, not everyone has the chance to curate the age gap between their kids.

For parents who do, it can be a tricky decision.

There are all kinds of studies you can read on the impact of sibling age gaps, including for the birthing parent, the children's development, and the quality of sibling relationships.

The research has its limitations, and on some of the outcomes considered, there are contradictory conclusions.

But there are findings that may help sway you one way or the other, depending on what you value most, as the experts we spoke to explain.

'Creating less of a demand on yourself'

Emily now has four children. Twins Eve and Oscar came five years after Liam.

It means she's been able to experience two different age gaps in her family.

With the first two so close together, she says they "always have each other — a little playmate for life."

But the exhaustion and stretched resources were a challenge.

"There isn't much space for one-on-one time [with each of the children]."

With the larger age gap, she sees the older kids acting as great role models for their twin siblings, taking pride in teaching them things.

"Watching them read with them, play games outside, pushing them on the swing is really beautiful."

And perhaps no age gap is the dream for some?

"I'm sure it's not the same for everyone but I think twins were almost easier," Emily says.

"They were in the exact same routine, slept the same time, ate the same things at the same time, same size clothes and nappies, shared a room."

Parenting author Justin Coulson — who has six children with age gaps ranging from 14 months to four-and-a-half years — says a central theme of the research is competition between children for parental resources.

And from what he has observed in his work with families, and with his own children, he says bigger age gaps tend to be better.

"Parents who have kids who are closer together in age tend to be a lot more tired and lot less able to invest fully and wholeheartedly because they are just exhausted and resources are stretched.

Dr Coulson says the idea that children close in age will be best friends is "family folklore".

"Yeah they might have a great playmate who is a similar age, but they also fight a hell of a lot more."

With that said, Dr Coulson makes clear that every family is different, and therefore this won't ring true for everyone.

"There are so many factors that contribute to whether a [smaller age gap] is easy or hard."

What the research says

Cara Goodwin is a child psychologist in the US who translates scientific research into understandable information for parents.

After analysing many different studies on sibling age gaps, she says there is no clear "ideal age gap".

But her overall translation says research suggests an age gap of "27 to 32 months may be associated with the best health outcomes for mother and child."

"In terms of sibling relationship, siblings who are closer in age tend to fight more but are also closer which may have positive benefits for social skills," Dr Goodwin writes.

"Parents who have children closer in age may be more stressed and more likely to have relationship problems."

Senior lecturer in psychology at University of the Sunshine Coast Rachael Sharman says while that does seem to be the sweet spot, the research is "not particularly compelling".

"It gives mum's body time to recover from pregnancy and breastfeeding… also in terms of sibling rivalry and play, that is considered the goldilocks zone," Dr Sharman says.

"But there are so many individual factors to consider.

"And we are talking about [in the context of] the Western world here, because there are obviously cultural differences."

What do you value most?

While many parents might be focused on creating siblings who will be friends, both our experts say there are more important factors to consider.

People really need to think about their resources, both money and time, Dr Sharman says.

"If you are stretched too thin, you set up a scenario for sibling rivalry."

Dr Coulson says the parents' wellbeing is primary, as is the quality of the relationship between the couple (if relevant).

"There is clear research that shows the arrival of a child is associated with a decrease in relationship satisfaction.

"So we really want to make sure that [the parents] — especially the birthing mother for who it is physically and psychologically substantial — is doing well."