One in 10 younger women who develop breast cancer first spot signs when they are pregnant or breastfeeding, according to new research.
The number of women aged 45 and under diagnosed with breast cancer is thought to be on the rise, with an estimated 5,600 new cases in this age group each year.
Studies have suggested that the longer women delay motherhood, the higher the chance they will develop breast cancer in their lifetime.
The average age of mothers in 2014 increased to 30, around four years higher than in the 1970s.
New research from Breast Cancer Care among 496 women aged 45 and under who developed breast cancer has found one in 10 spotted signs while pregnant or breastfeeding.
More than a third (39%) of women went through treatment when their youngest child was aged five or under.
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.
Female breast cancer is strongly related to age, with almost half of all cases among those aged 65 and over.
Rates rise steeply from around age 30 to 34, level off for women in their 50s, then rise further at age 65 to 69.
Rates drop slightly for women aged 70 to 74 and then increase steadily to plateau in those aged 85 and over.
Up to age 29, women have a one in 1,950 chance of developing breast cancer, rising to one in 210 by age 39 and one in 48 by age 49.
Amanda Mealing, Breast Cancer Care ambassador, actress and mother-of-two, was diagnosed with breast cancer aged 34, the day after her second son was born.
She said: "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.
"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."
Samia al Qadhi, chief executive of Breast Cancer Care, said: "Being told you have breast cancer when you have a young family is devastating.
"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.
"A breast cancer diagnosis during pregnancy or soon after giving birth can mean some women find it difficult to bond with their baby.
"The feelings of guilt can be huge and they may feel they have nowhere to turn. So access to support is vital.
"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."
Official figures from the Office for National Statistics (ONS) show that just over 10,000 women are diagnosed with breast cancer under the age of 50 every year in the UK.
Breast Cancer Care estimates that 10% of all breast cancers diagnosed each year in the UK are in women aged under 45.