Some parents find school gates and toddler groups to be competitive arenas, a survey suggests.
Research carried out for The Baby Website among 3,000 parents found 40% disliked some of the "friends" they had made through their children.
The researchers say on average a woman makes 11 new friends through baby groups or at the school gate.
A major reason given for disliking such "friends" was competitiveness over children's development.
Another was "they think they know everything about raising children".
Does this mean a new species called "Mumzillas" is stalking the playgrounds of the UK or just that some natural parental pride has been taken the wrong way?
The co-founder of The Baby Website, Kathryn Crawford, thinks it is probably the latter, mixed with a liberal dose of parental insecurity.
"Sometimes I don't think mums are boasting; they are just proud of their children.
"It all starts with the births, when women get together afterwards and compare how long their labours were.
"Then it moves on to what their children can do and the insecurities start to creep in. We all read the books about the range of ages when children do different things but it does not stop us worrying.
"'Why can't my child crawl yet when that one can? When will mine smile? Why hasn't my daughter started reading Shakespeare at five?'"
Author Jojo Moyes said having children can turn parents into rivals.
Writing in the Daily Mail, she said she had made fantastic life-long friends while raising her three children but had also come across many women who were very judgmental.
"Part of the problem is that over the past decade parenting has become more prescriptive than it has ever been. There are competing manuals to tell you what to feed, how to train your baby to sleep," she said.
Competition raged around everything from how long someone breastfed their child to children's routines and diets and the activities toddlers were taken to.
The writer said she had once been congratulated for deciding to work from home by another mother who said she would now be "a proper mother".
"It took me about an hour to realise that I had just been kicked in the teeth by a Mumzilla."
Psychologist and parenting coach Jennifer Leonard says parents may be seen to be competing at the school gates because they mention the many activities they take their children to.
"Parents can be quite pushy and some children have such busy schedules. It's a very competitive world and some people are afraid of not pushing their children.
"They worry: 'Is my child going to make it? My child must succeed at everything'."
Bath-based Dr Leonard added: "A lot of parents I see are quite middle class with children at private schools and there it gets very competitive.
"Every holiday children are flying off to South Africa or Disneyland."
Kathryn Crawford, who is Cardiff-based, says although mothers everywhere can feel insecure, competitiveness is more of a phenomenen in south-east England.
"You hear of mothers feeling insecure about what they are wearing, what car they drive, but it was not like that for me," she says.
"My children went to a lovely, traditional primary school. There was nothing like the animosity and back-stabbing talked of."
The survey was carried out online for The Baby Website by One Poll in April.