The number of women in this age group diagnosed with breast cancer is thought to be "on the rise", with an estimated 5,600 new cases each year, suggests research from Breast Cancer Care.
The study looked at 496 women aged 45 and under who developed breast cancer. It also found more than a third (39%) of women went through treatment for the condition when their youngest child was aged five or under.
"Being told you have breast cancer when you have a young family is devastating," said Samia al Qadhi, chief executive of Breast Cancer Care, according to PA.
"Many mums feel they miss out on precious time with their children because they are going back and forth to hospital for treatment or may be dealing with debilitating side-effects - time they will never get back," said al Qadhi.
"We urge breast cancer units to adopt our recommendations for supporting younger women with breast cancer, which include a referral to a specialist if diagnosed during pregnancy."
In the research, half (53%) of 196 women who had young children felt the biggest impact of treatment was being too ill to care for them.
The biggest fear for 66% of mothers was not seeing their children grow up.
Amanda Mealing, Breast Cancer Care ambassador and mother-of-two, was diagnosed with breast cancer aged 34, the day after her second son was born.
"Breastfeeding my day-old son Otis for the first time should have been special, but instead as I felt a lump in my breast I realised something was very wrong," she said.
"When he was only days old I was told I had breast cancer. I was heartbroken. We had only just welcomed him into the world and now I was consumed by the fear I wouldn't see him or his brother grow up.
"Throughout treatment I faced sickness and extreme fatigue all while trying to be there for my kids – it was totally overwhelming."
The charity is urging cancer units to give extra support to women aged 45 and under diagnosed with breast cancer during pregnancy, both during and after pregnancy.
For more information on treatment of breast cancer during pregnancy, visit Cancer Research UK's guide.