Family History and Bowel Cancer Risk

Does Family History Really Matter?

Yes — it genuinely does. Around 20 to 30% of bowel cancers occur in people who have a family history of the disease. If a parent, sibling, or child has been diagnosed with bowel cancer, your own lifetime risk is roughly double that of someone with no family history. The risk goes up further if more relatives are affected, or if the person who was diagnosed was young when it happened.

Understanding your family history is the starting point for knowing which screening pathway is right for you. Australia's National Health and Medical Research Council (NHMRC) has published clear guidelines that divide people into three risk categories based on family history — each with specific guidance on when to start screening and how often. You do not have to figure this out on your own; your GP can help you work out which category applies to you.

The Three Risk Categories

The table below summarises the NHMRC risk categories and what each one means for your screening. Use this as a starting point — then talk through your own situation with your GP or a colorectal surgeon, because the details of your family history matter.

Moderate Risk: One Close Family Member Has Had Bowel Cancer

If one of your parents, brothers or sisters, or children has been diagnosed with bowel cancer — at any age — you are in the moderate risk category. Your lifetime risk is roughly twice that of someone with no family history. That sounds worrying, but moderate risk also means there is a clear, well-established plan to keep you safe.

The NHMRC guideline is to start colonoscopy screening from age 40 to 45, or from 10 years before the age your relative was diagnosed — whichever comes first. For example, if your father was diagnosed at 52, you would aim for your first colonoscopy around age 42.

If your colonoscopy is clear, the recommendation is to repeat it every 5 years. If any polyps are found, your specialist will advise how soon to come back based on what type and how many were found.

High Risk: Multiple Relatives or an Early Diagnosis in the Family

You are in the high risk category if:

• Two or more close relatives (parents, siblings, or children) have had bowel cancer
• A close relative was diagnosed before the age of 55
• Bowel cancer or polyps have appeared across more than one generation of your family (for example, a parent and a grandparent)
• You or a close relative have been diagnosed with an inherited bowel cancer condition

If you are in the high risk category, colonoscopy usually starts between age 25 and 35 and is repeated every one to two years, depending on what is found and the full picture of your family history. Your exact schedule should be worked out with both a colorectal surgeon and, where it makes sense, a clinical geneticist. You do not need to carry this information alone — there is specialist support available to help you navigate it.

Hereditary Bowel Cancer Conditions

A small number of bowel cancers — around 5% — are caused by an inherited gene change rather than by a gradual build-up of lifestyle and environmental factors over time. The two most important conditions to know about are:

Lynch Syndrome

Lynch syndrome is caused by an inherited fault in certain genes (MLH1, MSH2, MSH6, or PMS2) that are responsible for correcting mistakes when cells divide. It is the most common hereditary bowel cancer condition, accounting for around 3 to 5% of all bowel cancers. People with Lynch syndrome have a lifetime bowel cancer risk of 40–80%, depending on which gene is involved. The condition also raises the risk of endometrial (womb), ovarian, stomach, and urinary tract cancers.

If you have Lynch syndrome, colonoscopy screening typically starts at age 25 and is repeated every one to two years. A simple blood test can confirm whether you carry the gene change. Testing is recommended for anyone with a strong family history or whose tumour shows specific features on pathology.

Familial Adenomatous Polyposis (FAP)

FAP is caused by a change in a gene called APC. People with FAP develop hundreds — sometimes thousands — of polyps throughout the colon, usually starting in their teenage years. Without intervention, bowel cancer becomes almost inevitable by age 40 to 50. Surveillance usually begins in the early teens, and removing the colon (called a colectomy) is typically recommended in early adulthood to prevent cancer from developing.

There is also a milder version called Attenuated FAP (AFAP), which causes fewer polyps and tends to appear later in life. Another related condition, MUTYH-associated polyposis (MAP), is inherited differently but also causes multiple polyps and a raised cancer risk.

What About More Distant Relatives?

Second-degree relatives include grandparents, aunts, uncles, and half-siblings. A single second-degree relative with bowel cancer does raise your risk a little, but not enough on its own to trigger early colonoscopy screening. However, if bowel cancer appears across multiple second-degree relatives — especially at a young age — a genetics assessment is worth discussing.

If you are not sure whether your family history is significant enough to act on, bring it up with your GP. They can help you assess the picture and refer you to a familial cancer clinic if a formal risk assessment or genetic counselling is warranted. You do not need to arrive knowing all the answers.

What to Do If You Have a Family History

The most important thing is to speak to your GP. Bring as much detail as you can about the relatives who have been affected: what their diagnosis was, how old they were at the time, and whether any genetic testing was done in the family. Your GP can then work out which risk category fits your situation and refer you to a colorectal surgeon if a colonoscopy is the right next step.

Frequently Asked Questions