Like many new mums, Rachael Casella frequently finds herself gazing at her baby, overwhelmed with happiness.
"Sometimes I feel like I'm going to explode with love," she says, smiling. "Izaac is amazing and the past few months have been incredible."
But these precious moments with the now-five-month-old little boy are also bittersweet for the Sydney mum.
"Although it's a really beautiful time, it's complex. I feel guilty because maybe I shouldn't be happy then I feel guilty for feeling that too," admits Rachael, 37.
It's no surprise Rachael's emotions are in flux. Nearly four years ago, she and husband Jonny's beloved little girl Mackenzie passed away at seven months and 11 days old.
Mackenzie had spinal muscular atrophy type 1 (SMA), the terminal genetic condition that progressively destroys nerve cells that control breathing, swallowing and moving. She was diagnosed at 10 weeks old.
"It's been a really hard few years but I honestly believe that Mackenzie chose Izaac specifically for us," Rachael tells Woman's Day.
"And I feel like he's exactly what we needed – he's incredible."
Izaac's arrival in March was a moment the couple feared might never come.
Since losing her daughter, Rachael endured multiple pregnancy losses, surgeries and nine rounds of IVF.
But even when the pregnancy was quite far along, having been tested and scanned almost every step of the way and with confirmation Izaac didn't have SMA, Rachael was still terrified something could go wrong.
"We didn't trust that we were actually going to be able to keep him," she admits.
Now Izaac is here, the besotted mum – unsurprisingly – struggles to be away from him for more than a few moments.
"I rush to him quite quickly," laughs Rachael.
"I don't like seeing him upset. So I'm just trying to find that balance between snuggling him all the time and being able to put him down. It's a struggle!"
But while family life is full of special moments with a newborn to cuddle, the memory of Izaac's big sister is never far from Rachael's thoughts.
"He's really smiley – it's his new skill, and he's like Kenzie... he smiles with his eyes," she says.
"He's also developing a little dimple in the exact same spot she had, which is really beautiful."
In between snuggles, feeds and nappy changes, Rachael – who, like her husband is a federal police detective – is working to ensure Mackenzie's legacy helps other families avoid heartache like theirs.
After learning Mackenzie had SMA because she and Jonny were, unknowingly, both genetic carriers, Rachael began lobbying for greater testing for prospective parents.
The result is Mackenzie's Mission, a $20 million screening study.
The aim is that all prospective parents will one day be offered free reproductive genetic testing, carried out with a simple saliva swab.
Rachael also hopes the routine heel prick test given to all newborns will begin screening for SMA, too.
"SMA is the number-one genetic killer of babies but it's not screened for," explains Rachael.
"It's a motor neuron condition so you slowly lose ability to move and you can't get that back. So it's really important to give the treatment as soon after birth as possible."
Closer to home, Rachael and Jonny are adamant Mackenzie remains part of their everyday life.
They have photos of her throughout their home, keep a candle lit in her memory and make sure to talk to friends, family and Izaac about her all the time.
"She's in everything but it's trying to find a real balance for us to keep her around as our daughter and his big sister, but also to make sure Izaac doesn't feel overshadowed and can be his own person," explains Rachael.
"But I hope that when he's in school and asked to draw a picture of his family, Mackenzie's in there, too."
Visit mackenziesmission.org.au and @mylifeof_love