We appreciate Dr. Kamphuis et al’s and Dr. Ricci et al’s interest in our article.¹ We agree that male factor infertility incorporates a range of disease severity. Findings from a recent study indicate no association between male factor infertility and perinatal outcome.² The rate of intracytoplasmic sperm injection use in our study was not surprising; in 2010, intracytoplasmic sperm injection was used in 66% of fresh, nondonor in vitro fertilization cycles in the United States, regardless of infertility diagnosis.³ We agree that surveillance data are limited by the lack of some historical medical information, and this was noted as a limitation in the article. The aim of this observational study was not to suggest causality but to evaluate a potential association. Although we were unable to separate iatrogenic from spontaneous preterm birth, we did stratify by early-preterm and late-preterm birth because most iatrogenic preterm birth occurs at later gestational ages. After stratification, the association between tubal factor infertility and preterm birth remained. Although propensity scores could have been used to account for possible confounding by obstetric history and maternal age, its use would have reduced the sample size, depending on the closeness of the matching algorithm. We considered regression analysis to be preferable because it is an established and accepted tool for evaluation of confounding and allowed us to capitalize on the large study population.

We maintain that the decrease in the percentage of total assisted reproductive technology cycles holding a tubal factor diagnosis reflects increased alternative indications such as diminished ovarian reserve. Although Chlamydia trachomatis rates have increased, hospital admissions for pelvic inflammatory disease have decreased.⁴ Pelvic inflammatory disease, rather than chlamydial infection, may explain more accurately the decrease in absolute number of tubal factor cycles. Regarding the potential effect of selective reduction, our analysis demonstrated an association between poor perinatal outcome and tubal disease among singleton pregnancies after controlling for number of fetal heartbeats at first ultrasound scan.

Our study should be interpreted with care and is subject to several limitations. Future studies that include additional preterm birth risk factors may help to delineate the association between tubal disease and preterm birth.