Grand Rounds

Does Duration of Pregnancy Impact Endometrial Cancer Risk?

BACKGROUND AND PURPOSE:

Increased parity is associated with a reduction in risk of endometrial cancer
- Mechanism regarding this protective effect is unclear
Husby et al. (BMJ, 2019) examined the association between pregnancy duration and endometrial cancer risk to determine if whether simply being pregnant is protective or length of time one is pregnant is the protective factor

Nationwide cohort study
Participants
- All Danish women born from 1935 to 2002
Data analysis
- Relative risk (RR) estimated using log-linear Poisson regression
- Adjustments made for following confounders: Pregnancy history | Educational attainment | Marital status | Socioeconomic variables
Primary outcomes
- Relative risk of endometrial cancer by pregnancy number, type, and duration

2,311,332 women | 3,947,650 pregnancies | 57,347,622 person years of follow-up
- Endometrial cancer: Diagnosed in 6,743 women
First pregnancy associated with reduced cancer risk
- Induced abortion: adjusted RR 0.53 (95% CI, 0.45 to 0.64)
- Childbirth: RR 0.66 (95% CI, 0.61 to 0.72)
Each subsequent pregnancy was associated with an additional risk reduction
- Induced abortion: RR 0.81 (95% CI, 0.77 to 0.86)
- Childbirth: RR 0.86 (95% CI, 0.84 to 0.89)
- Each pregnancy from the second onward had a similar protective effect
The following did not impact the results
- Duration of pregnancy | Age at pregnancy | Spontaneous abortions | Obesity | Maternal birth cohort | Fecundity | Socioeconomic factors

Strong protective effect of first pregnancy and risk for endometrial cancer
Endometrial cancer risk is reduced even if pregnancy ends shortly after conception (induced abortion similar to pregnancies ending in childbirth)
Authors acknowledge that results diverge from ‘unopposed estrogen theory’ of endometrial cancer
- Data suggests some effect early in pregnancy (perhaps elevated progesterone levels play a protective role)

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