Age at First Pregnancy, Adult Weight Gain and Postmenopausal Breast Cancer Risk: The PROCAS Study (United Kingdom)

Area of Science:

• Reproductive Health
• Oncology
• Epidemiology

Background:

• Adult weight gain (AWG) is a known risk factor for postmenopausal breast cancer (BC).
• An early first pregnancy (FP) is associated with reduced BC risk.
• Pregnancy is a significant factor in weight changes, prompting investigation into the interaction between AWG and FP on BC risk.

Purpose of the Study:

• To investigate the interaction effect between age at first pregnancy and adult weight gain on breast cancer risk.
• To identify high-risk groups for targeted breast cancer prevention strategies.

Main Methods:

• Prospective cohort study (Predicting Risk of Breast Cancer at Screening - PROCAS).
• Analysis of data from 48,417 women recruited between 2009-2015.
• Cox proportional hazards model to assess interaction between FP age and AWG on BC risk, with a median follow-up of 6.4 years.

Main Results:

• 1702 incident breast cancers were identified.
• Highest risk observed in women with substantial AWG (>30%) and late FP (≥30 years) (HR: 2.48) or nulliparity (HR: 2.38) compared to early FP (<30 years) and stable weight (≤5%).
• Elevated risk was noted even with moderate weight gain (5%-15%).
• A positive trend for additive interaction was seen with late FP (RERI: 0.32), while risk in nulliparous women appeared independent of weight gain (RERI: -0.05).

Conclusions:

• Stable adult weight and early first pregnancy are independently protective against breast cancer.
• Adult weight gain is a significant risk factor irrespective of reproductive history.
• The combination of substantial AWG with late or no pregnancy identifies a high-risk group for breast cancer, warranting prioritization for weight management interventions.