The incidence of breast cancer in young women is lower in developed countries [7] in comparison with developing ones.

However, the incidence of breast cancer among young women in the low-risk population of the South Asian countries is higher [8].

Data on the temporal trends of breast cancer incidence in young women are scant and limited [9].

Some recent studies have suggested that the incidence of breast cancer in young [4, 10, 11] and premenopausal women [12, 13] increased in Europe and the USA.

A study from different countries of Europe showed an overall increase of 1.19 % per year from 1990 to 2008 [11]. The incidence increased more consistently among women under 35 compared to the 36–40 years cohort.

A study from France reported an increase during the period of 1983–2002 [10].

Similar findings also emerged from analysis conducted in Asian countries. An analysis including data from tumor registries in China, Japan, Singapore, and the Philippines considering the calendar period 1970–2002 showed that the trend in incidence for the age group 16–29 increased from 0.45 to 1.07 corresponding to an AAPC of 2.8 % (95%CI 1.9, 3.7). In age group 30–40, the incidence ranged from 13.3 in year 1970 to 24.8 in year 2002 corresponding to an AAPC of 2.7 % (95 % CI 2.3, 3.1). There were two statistically significant changing points in the regression line for the age groups 30–40 and 16–40: one point in the year 1975 with an APC of 6.1 (5.1, 7.1) and the other in 1985 with an APC of 0.4 % (0.01) [13].

Considering developing countries, a study conducted in Brazil reported an annual increase of 5.22 % in the incidence of breast cancer among age group 30–40 years during the period 1988–2003 [14].

Different trends have been observed in the same country in different ethnic groups.

For example in the USA, white women who were 40 years or older had a higher rate of breast cancer than black women in the same age group, but among younger women black women had higher rates. Otherwise, the annual percentage change in invasive breast cancer incidence increased only among white women younger than 40 years.

Other racial and ethnic groups had lower incidence rates than non-Hispanic white women for all three age groups and did not exhibit the crossover pattern observed among black women, although IRRs were slightly higher among younger than older Hispanic, API, and AI/AN women.

Any change in trend of any disease, especially breast cancer, can be attributed to either a true increase due to an underlying change of some risk factors, the result of an improvement in the quality of data collection or screening practice.

Along this line, part of these different trends could be explained by different frequency among countries and ethnic group of screening procedures. A younger age at onset among the Asian population has been reported and it has been attributed to a cohort effect that has been decreasing in recent decades [15].

Among young women, a specific topic is the incidence of the disease during pregnancy or in women who have recently delivered.

Breast cancer is generally reported to be the most common pregnancy-related cancer in all the populations. It accounts for about one-third of all pregnancy-related cancers. Its incidence mirrors the incidence of breast cancer among nonpregnant women of the same age class. In developed countries, the risk of breast cancer diagnosis in pregnancy or during the first year after delivery is about 1:3,000 pregnancies.

It is commonly reported that young breast cancer patients are diagnosed with more advanced stages.

In fact, breast cancer cases in young women have larger tumors, more frequent nodal involvement, and are more likely to be diagnosed with stage II or III cancer than older patients [17].

This may be at least in part due to the fact that younger women are not screened for breast cancer. In fact, mammography is not recommended in women less than 40 years.

Further, young women are often less likely to seek early medical advice, leading to later detection often at more advanced stages [18].

Pregnancy is another reason to delay diagnosis, since pregnant women may tend underreport breast nodules.

The tumors diagnosed in young women have different characteristics than those in older ones. Young women have a lower rate of ductal carcinoma in situ. This may be due to a detection bias, since, as previously underlined, women under 40 years of age do not have screening mammograms.

Further, tumors in young women are more likely to be high grade and to have high proliferation index.

Among young women, lymphovascular invasion is more common than among older ones [19].

Many studies have shown that young women with breast cancer have more commonly hormone receptor (HR) negative tumors. Otherwise, HER2 is overexpressed [3].

Further, young women are likely to be diagnosed with HR+/HER2- subtype of breast cancer, followed by triple negative, HR+/HER2+, and HR-/HER2+ subtypes. Multiple studies demonstrated the overrepresentation of triple-negative breast cancers in young patients particularly in African Americans [1].

In a retrospective study of 700 breast tumors conducted by Anders et al. women younger than 45 years were less likely to have estrogen receptor-positive disease, and more likely to have grade 3 tumors, nodal metastasis, and larger primary breast tumors [20].

The incidence of breast cancer is the highest in the non-Hispanic white population. When stratified by age, incidence rates are similar for non-Hispanic whites and African Americans between the ages of 30 and 49 years. However, in patients younger than 40 years specifically, African American females have the highest relative incidence of breast cancer [27, 28].

In women over 45, breast cancer is more common in whites than blacks. However, black women under age 35 have more than twice the incidence of invasive breast cancer and three times the breast cancer mortality of young white women [6].

In contrast, Native American women aged 20–44 have a lower incidence of breast cancer (relative risk [RR] = 0.7) compared to the general population.

A positive family history of breast cancer is the main risk factor among young women. Women under 35 years of age with a positive family history of the diseases have 3–4 times higher risk of the disease than women without family history [29].