Mammogram Screening: When to Start and How Often in Canada and the US

You are turning 40, or you are in your 40s, and you hear different advice about mammograms. A friend in the US started at 40. Your clinician in Canada may have said 50. Both can be right for their setting. This guide explains what each country's guidance says, why it differs, and what to weigh for yourself.

What a screening mammogram does

A mammogram is a low-dose X-ray of the breasts. A screening mammogram is for people with no breast symptoms. It looks for cancers too small to feel [3].

Most screening now uses digital mammography. Many centres also use 3D mammography (digital breast tomosynthesis), which takes images from several angles.

A diagnostic mammogram is different. It is used when there is a lump, pain in one spot, skin change, or nipple discharge, or to follow up an abnormal screen.

What your clinician checks: the guidelines

US guidance

The US Preventive Services Task Force (USPSTF) 2024 recommendation [1]:

• Ages 40 to 74: a mammogram every 2 years (grade B).
• Age 75 and older: not enough evidence to judge benefits and harms (an "I" statement).
• Dense breasts: not enough evidence to recommend extra ultrasound or MRI after a normal mammogram.

The 2024 update lowered the starting age from 50 to 40. The USPSTF cited rising breast cancer rates in women in their 40s and higher death rates among Black women [1].

Canadian guidance

The Canadian Task Force on Preventive Health Care's 2018 guideline recommended screening every 2 to 3 years from 50 to 74 and suggested not routinely screening in the 40s.

In 2024, the Task Force released draft updated recommendations [2]:

• Breast screening is a personal choice for women aged 40 to 74, who should get clear information on benefits and harms in absolute numbers.
• Ages 40 to 49: it suggested not systematically screening, but said those who want screening after being informed should be offered a mammogram every 2 to 3 years.
• Ages 50 to 74: screening every 2 to 3 years.

These draft recommendations were never finalized. The Task Force ended its mandate in March 2026, and the draft is now archived [2].

What happens in practice in Canada

Provincial and territorial programs set access. Several, including British Columbia, Ontario, and Nova Scotia, let people aged 40 and older book a screening mammogram without a referral. Most programs invite people aged 50 to 74 every 2 years. Rules vary, so check your own program.

Benefits and harms in plain numbers

Screening lowers the chance of dying from breast cancer. The benefit is real but smaller in the 40s than in the 50s and 60s, because fewer women in their 40s get breast cancer.

Harms are also real.

• False positives: a recall for more imaging that turns out to be normal. In a large US study, after 10 years of screening every 2 years starting at 40, about 42 percent of women had at least one false-positive recall. With yearly screening it was about 61 percent [4].
• False-positive biopsy: about 5 percent of women screened every 2 years for 10 years were advised to have a biopsy that showed no cancer [4].
• Overdiagnosis: some screen-found cancers, such as some cases of ductal carcinoma in situ (DCIS), would never have caused harm. Treating them adds risk without benefit.
• Radiation: the dose from a mammogram is low.

Screening every 2 years has fewer false positives than yearly screening, with most of the benefit kept [4].

Dense breasts

Breast density describes how much of the breast is glandular and fibrous tissue compared with fat. Dense tissue is common and normal. But it makes mammograms harder to read and slightly raises cancer risk [3].

In the US, mammogram reports must now tell you if your breasts are dense. In Canada, reporting varies by province. The USPSTF found that the evidence is not yet clear on whether extra ultrasound or MRI helps people with dense breasts [1]. Ask your clinician what is available and what it would mean for you.

Who needs a different plan

These guidelines are for people at average risk. You may need earlier screening, often with yearly MRI as well, if you have [5]:

• A known BRCA1, BRCA2, or other high-risk gene change.
• A very strong family history of breast or ovarian cancer.
• Radiation to the chest before age 30.
• A past breast cancer or some high-risk breast biopsy results.

Questions to ask your clinician

• Based on my risk, when should I start and how often should I go?
• What are the benefits and harms for someone my age, in absolute numbers?
• Can I book my own mammogram here, or do I need a referral?
• Are my breasts dense, and would extra imaging help me?
• Does my family history mean I need genetic counselling or MRI?
• When can I stop screening?

When to get care quickly

See your clinician soon, not at your next screen, for:

• A new lump or thickening in the breast or armpit.
• Skin dimpling, redness, or a change in breast shape.
• A nipple that turns inward or bloody nipple discharge.

Go to the emergency department for a hot, red, painful breast with fever, which can be an infection.

Bottom line

In the US, the USPSTF recommends a mammogram every 2 years from 40 to 74. In Canada, national guidance did not recommend routine screening in the 40s, but many provinces now let you choose it from 40. False positives are common, and dense breasts add uncertainty. Your personal risk and values should guide when you start.

Primary AI can show you the guideline and study sources behind this topic in plain language, so you can bring better questions to your appointment.