Neurofibromatosis type 1 (NF1) is associated with a high risk for cancer. Benign cutaneous neurofibromas of women with NF1 may increase in size and number during pregnancy. However, it is not known whether pregnancy affects the risk for cancer in NF1.

We retrieved the pregnancies of women in the Finnish NF1 cohort and in a 10-fold control cohort from the Finnish Medical Birth Register. Cancers occurring during or after pregnancy were obtained from the Finnish Cancer Registry and summarized using standardized incidence ratio (SIR). The cancer incidence of nonNF1 mothers of individuals with NF1 was also estimated.

Totals of 263 pregnancies in 136 women with NF1 and 3176 pregnancies in 1720 controls were observed. In the NF1 group, two cancers were identified during pregnancy and the year following the delivery (SIR 6.44, 95% CI 1.07–19.89). Among controls, the SIR was markedly lower (0.25, 95% CI 0.01–1.08). Within 1–10 years after pregnancy, the SIR of women with NF1 was 7.54 (95% CI 4.15–12.41). The SIR of women with NF1 aged 20–49 years, and without a known history of deliveries was 8.63 (95% CI 6.08–11.81). The nonNF1 mothers displayed a SIR of 0.81 (95% CI 0.66–1.00) after giving birth to a child with NF1.

The pregnancy-related cancer incidence in women with NF1 is similar to women with NF1 aged 20–49 years overall, although notably higher than in the general population. Giving birth to a child with NF1 does not affect the risk for cancer in women without NF1.