Take mum-of-one, Emma. She was in such pain after the complicated delivery of her daughter that she couldn't even sit down comfortably. With her husband back at work within a week and her parents working full time, she was confident her retired in-laws would support her.
After all, when her husband's sister became a mum, Emma's mother-in-law had cooked her meals, helped with housework and offered emotional support, so Emma was hopeful of the same.
However, after their initial visit to the maternity ward, her in-laws went away for 10 days on a last-minute holiday.
"I couldn't believe it," she says.
I always thought we had a really close relationship and here they were with a new grandchild they didn't want to see and a daughter-in-law who was an emotional and physical mess. They would never have left their own daughter like that.
Four years later, she notices her mother-in-law making a beeline for her daughter's child, rather then hers, at family gatherings. "I genuinely feel mothers prefer their daughters' children to their sons'," she says.
Jane, a mum of two, agrees. Her in-laws provide childcare for their daughter's children so that both parents can work – but they don't do the same for their son's children.
"Admittedly, my sister-in-law lives around the corner from them, and we're a 15 minute drive away, but it's only 15 minutes, and it means we have to pay for childcare and they don't."
Psychologist Professor Peter Smith, of London's Goldsmiths University, agrees that while grandparents generally say they are equally close to all grandchildren, research shows they are, in fact, closer to their daughters' children than their sons'.
He offers two reasons. "The first is that mothers tend to be closer to their daughters and the second is that of paternity," he says.
"Certainly they can be sure that their daughters' children are theirs, but they can't be quite so sure with their sons' children. They may not be consciously aware that this is an issue but it factors in research studies."
Mother-of-one Natalie thinks it's because sons often place less importance on the involvement of the extended family. Admittedly, she lives in the same town as her parents, whereas her brother and family live two hours away, but she also makes sure her parents know what is going on in her son's life with regular phone calls and messages.
"My parents have more of a connection with my son simply because they see him more and importantly get to hear about what is happening in his life very regularly. It's not that they love their granddaughter any less but there is that distance there, both geographically and, in a sad way, emotionally because there isn't that constant steam of information."
Sue, a mother of three and a grandmother of four, says she loves her grandchildren equally but is closer to her daughter's children because she cares for them two days a week, whereas her son's children are cared for by their maternal grandmother.
"My son is very conscious that I'm so close to his sister's children," she says. "He makes sure I don't give more attention to my daughter's children than his. Because I see such a lot of them, he feels his children are pushed into the background – but they are not at all.
"When I started minding my daughter's children, I told him I was only doing for my daughter what his mother-in-law is doing for her daughter."
Perhaps mums are guilty of excluding their mothers-in-law in favour of their own mums. Maria, mum of two boys, says, "I definitely prefer my parents to my husband's, and naturally have a closer relationship with them.
"I speak to my mum every day whereas he speaks to his mum once a week and so as a result mine are more involved."
Both sets of grandparents live two hours away in the same town, yet when Maria and her family visit, she insists on staying with her parents because that is where she is more comfortable.
Wanting to replicate our parents' parenting methods could mean we turn more to mothers than mothers-in-law.
Indeed, a recent study by Pampers found that six in 10 women turn to their own mother for advice on getting baby to sleep.
As Maria says, "It helps that my mum is a health visitor and breast feeding counsellor, and breast-fed me and my brothers so has influenced me and is very supportive of the parenting choices I've made.
"My husband's mum is more old fashioned and bottle fed him and I often feel she thinks I'm a bit odd with my home birth, cloth nappies, breast feeding and baby-led-weaning."
And it's not just grandmothers who seemingly show preferences. Lucy, mother to one son and one daughter, wonders if grandfathers prefer grandsons. "I have got embarrassed in the past as my dad dotes on my son and gets told by my mum when we are all together to give more attention to the other grandchildren," she says.
Mum of two Kate adds, "My brother is definitely my paternal grandfather's favourite. Perhaps it's because he was the first grandson after three granddaughters; the first guaranteed to carry on the family name."
Despite her sons being just three years old and six months, Maria admits to already worrying about being a grandmother. "I worry that when my sons have children I might end up being excluded as the paternal grandmother. I know it doesn't always work that way and often depends on distance and location, but it's crossed my mind. Although I've always wanted three children I guess part of me is hoping number three will be a girl."