We examined the association between maternal breast cancer and adverse birth outcome in a nationwide cohort and found little difference in the occurrence of preterm birth, low birth weight at term, stillbirth, or congenital abnormalities, compared with the comparison cohort, among newborns of women who were diagnosed with breast cancer before pregnancy.
The eight-fold increased odds of preterm birth for newborns of women who were diagnosed with breast cancer during their pregnancy reflected a higher rate of elective early delivery, probably to allow an earlier start of cancer therapy. After adjustment for gestational age, there was a 240
g reduction in mean birth weight for newborns in this group. The association with preterm birth in Group 3 may be explained by suboptimal intrauterine conditions caused by a preclinical cancer. In this group, only boys had increased odds of low birth weight at term, suggesting that male foetuses are more vulnerable than female.
Our data are derived from a uniformly organized health care system with complete cancer and birth registration. Some selection problems are possible, however. If women with breast cancer had more miscarriages or induced abortions caused by foetal abnormalities than comparison mothers, this phenomenon could explain why we found no increased risk of congenital abnormalities. It has been suggested that exposure to severe periconceptional life events might reduce the male proportion of offspring, partly because of differential abortion of male embryos (Hansen et al, 1999
). Thus, a lower proportion of males for offspring of the patients could be an indicator of miscarriages. Another study has indicated an increased risk of miscarriage among women with breast cancer (Velentgas et al, 1999
). Our data, however, did not show any important difference in male proportions between the offspring of breast cancer women and offspring of comparison mothers. It has been reported that women with high socioeconomic status have a higher incidence of breast cancer (Danø et al, 2004
), while low socioeconomic status has been associated with adverse birth outcome (Luo et al, 2004
). We were unable to adjust for socioeconomic status and therefore we may have underestimated the effect of the disease.
A recent study found that treatment data recorded in the Cancer Register are of varying quality (Jensen et al, 2002
). However, breast cancer treatment with surgery alone was correctly registered for 95.4% (Jensen et al, 2002
). Coding mistakes are infrequent in the Birth Registry, but data have some misclassifications of gestational age (Kristensen et al, 1996
). Our data did not suggest any differential misclassification of preterm birth between women with breast cancer and comparison mothers.
Hospital discharge data are not always coded correctly (Larsen et al, 2003
), but Danish data on congenital abnormalities are of high quality compared with other countries, with 80–85% coded correctly (Larsen et al, 2003
). We did not find any clusters of congenital abnormalities in any specific organ system.
Our finding of an increased risk of giving birth preterm for women who were diagnosed with breast cancer during or shortly after pregnancy corroborates the results of two earlier studies (Zemlickis et al, 1992
; Smith et al, 2001
). In a hospital-based study, Smith et al (2001)
identified 423 cases of breast cancer diagnosed from 9 months preceding delivery until 12 months after delivery over a period of 6 years in California. They reported an odds ratio of 2.2 (95% CI=1.7–2.8) for prematurity, and an odds ratio of 2.0 (95% CI=1.0–4.1) for very low birth weight. They adjusted only for maternal age. A hospital-based historical cohort study from 1992 of 118 women, who were pregnant within 9 months before or 3 months after their first treatment for breast cancer, reported a lower mean birth weight after adjustment for gestational age and a higher proportion of preterm births among offspring of women with breast cancer compared with controls (Zemlickis et al, 1992
). In these studies, however, the authors did not distinguish between birth outcome of women diagnosed with breast cancer during their pregnancy and women diagnosed shortly after pregnancy. We found a lower mean birth weight limited to newborns of women diagnosed during their pregnancy.
In conclusion, this is the first population-based cohort study of birth outcome in women diagnosed with breast cancer before pregnancy, and the largest cohort study to date of birth outcome in women diagnosed with breast cancer during or shortly after pregnancy. Overall, our results are reassuring regarding the risks of adverse birth outcome for women with breast cancer.