MINNEAPOLIS – Pregnancy-associated cancers are increasing, although the phenomenon of delayed childbirth is only partially responsible, researchers suggest.
From 1994 to 2008, the crude incidence of pregnancy-associated cancer increased from 112 to 192 per 100,000 pregnancies (P less than .001) in an analysis of 787,907 Australian women.
During the same period, the number of Australian mothers aged 35 years or more nearly doubled from 13% to 24%, including an increase from 2% to 4% of mothers over age 40, Christine L. Roberts, Ph.D., said at the annual meeting of the Society for Pediatric and Perinatal Epidemiologic Research.
After the cancer rate was standardized to the age of the 1994 population, however, only 14% of the increase in cancer was explained by increasing maternal age, said Dr. Roberts of the University of Sydney, New South Wales, Australia.
"Improved diagnostic techniques, detection, and interracial health services likely contribute to the unexplained portion," she said. "The increasing incidence of cancer confirmed a clinical impression that obstetricians were seeing women with cancer more frequently, although of course it remains uncommon."
The growing number of women postponing childbearing has raised concerns that the incidence of pregnancy-associated cancer would rise. The incidence is generally reported to be about 1 in 1,000 pregnancies, but estimates based largely on cancer reports have been imprecise, Dr. Roberts said.
The investigators obtained cancer and maternal information from linked cancer registry, birth, and hospital records for 1.31 million pregnancies and 1.33 million infants among 781,907 women in Australia.
During the study period, 1,798 women had a new cancer diagnosis: 499 during pregnancy and 1,299 within 12 months of delivery. This equates to 137.3 cancers per 100,000 pregnancies, Dr. Roberts said.
There were 42 cancer deaths, or 3.2 deaths per 100,000 pregnancies.
The highest proportion of cancers (14.5%) was diagnosed in the first 2 months post partum, lending support to the rationale that women and physicians may incorrectly attribute cancer-related symptoms to the physiologic changes of pregnancy and may be reluctant to use radiographs or invasive procedures during pregnancy, she observed.
The cancers were predominantly melanoma (599) or breast cancer (377), followed by thyroid/endocrine (228) and lymphohematopoietic (151) cancers.
Melanoma was twice as likely to be observed in pregnant women as in women of similar reproductive age (observed to expected ratio, 2.2), according to the authors, led by Dr. Yuen Yi (Cathy) Lee of the New South Wales Ministry of Health in North Sydney, Australia.
In prior studies, breast and thyroid cancer were the most common pregnancy-related cancers in California in the 1990s (Am. J. Obstet. Gynecol. 2003;189:1128-35), whereas more recently, melanoma and cervical cancer were the most common cancers during pregnancy in Norway (J. Clin. Oncol. 2009;27:45-51), Dr. Roberts noted.
In logistic regression analysis adjusted for age, country of birth, socioeconomic status, rural residence, parity, plurality, previous cancer, and assisted reproductive technology, significant risk factors for a pregnancy-associated cancer were previous cancer diagnosis (adjusted odds ratio, 3.8), multiple pregnancy (OR, 1.5), age 30-34 years (OR, 2.1), age 35-39 years (OR, 3.0), and age 40 years or older (OR, 3.6).
Women with a cancer diagnosis had a significantly higher risk of thromboembolic events (OR, 10.2), sepsis (OR, 4.3), and life-threatening maternal morbidity (OR, 6.9) after adjustment for maternal age, socioeconomic status, plurality, parity, previous preterm birth, diabetes, and hypertension.
A novel finding was that cancer during pregnancy also was associated with large-for-gestational age infants (OR, 1.5), said Dr. Roberts, who pointed out that large-for-gestational age is also a risk factor for pediatric cancer.
"Elevated levels of maternal hormone angiogenic factors during pregnancy may influence both infant size and tumor growth," she speculated.
Dr. Roberts said there is an Australian national policy on cervical screening recommending that Pap smears be offered to every woman presenting for antenatal care who has not had cervical screening within the past 2 years; however, this was introduced in 2008 at the end of the study period. "We are not aware of other policies for screening during pregnancy," she added.
Full details of the study are expected to be published in the coming weeks (BJOG 2012 [doi: 10.111/j.1471-0528.2012.03475.x]).
The authors report no conflicts of interest.