Increasing incidence of cancer during pregnancy, yet maternal and perinatal risks are low

Cancer during pregnancy is a challenge for the patient and treating doctors. In recent years several international studies have reported on outcomes in women and children in such pregnancies, yet because of its rarity most studies have been small and inconclusive. In three new studies from Karolinska Institutet, researchers combined the Swedish Medical Birth Register with the Swedish Cancer Register and the National Quality Register for Breast Cancer to identify women with cancer diagnosed during pregnancy and within the first year after delivery from 1973 to 2017. These studies are among the largest to date in this field and included 4629 women with pregnancy-associated cancer.

Reassuring findings for women and treating doctors

They found that the incidence of cancer during pregnancy increased in Sweden, with breast cancer, cutaneous malignant melanoma and cervical cancer being the three most common cancer types. In 2015-2017, cancer was diagnosed during pregnancy in around 50 per 100,000 deliveries while 110 per 100,000 were diagnosed during the first year after delivery. The ongoing trend of postponing childbearing to higher ages explained part of this increase, yet other risk factors remain to be identified.

In two studies focusing on breast cancer, the researchers found that breast cancers diagnosed during pregnancy and within one year after delivery had more adverse tumour characteristics, such as advanced stage and triple-negative subtype. Yet, importantly the survival was not different in these patients compared to other breast cancer patients with the same tumour types. The only exception was women diagnosed during the second trimester of pregnancy where a modestly increased mortality was found.

Women diagnosed with breast cancer had higher risks of preterm birth and perinatal outcomes related to preterm birth, such as low birth weight, yet no other risks such as maternal pregnancy complications or adverse outcomes in the newborn children were found. 

“Planned preterm birth is sometimes necessary to enable onset of cancer treatment, yet it is reassuring for both the women and the treating doctors that no other risks were identified in these women or in their children”, says Anna Johansson, research group leader at the Department of Medical Epidemiology and Biostatistics, Karolinska Institutet. “Due to the rarity of cancer during pregnancy many cancer doctors will only experience a handful of cases over their career. This shows the importance of undertaking clinically relevant studies using register data and high-quality epidemiological methods in close collaboration with clinicians.”

These studies were funded by the Swedish Cancer Society, Radiumhemmet Research Foundations and the Swedish Breast Cancer Association. There are no reported conflicts of interest.