Introduction: Multiple pregnancies are a recognized adverse effect of assisted reproductive technologies; nevertheless,
there is no consensus on the incremental risk associated with the ovarian stimulation (OS) used alone and intrauterine
insemination (IUI). The relationship between OS and IUI and the risk of major congenital malformations (MCM) is
Objective: To summarise the literature and evaluate the risk of multiple pregnancy and MCM associated with OS used
alone and IUI used with or without OS compared to natural conception (spontaneously conceived infants without any type
of fertility treatments).
Methods: We carried out a systematic review to identify published papers between 1966 and 2014 in MEDLINE, EMBASE
and the Cochrane Central Register of Controlled Trials. We included observational studies and randomized clinical
trials related to the risk of multiple pregnancies and MCM conceived following OS alone or IUI compared to natural conception
(spontaneously conceived infants without any fertility treatments). The quality of the included studies was evaluated
using The Cochrane Collaboration’s tool for assessing risk of bias for RCTs and the Newcastle–Ottawa Scale for observational
Results: There were 63 studies included in this review. Our systematic review suggests that the use of any OS alone was
associated with an increased risk of multiple pregnancy compared to natural conception (pooled RR 8.80, 95% CI 5.09-
15.20; p= 0.000; 9 studies). Similar increases in the risk of multiple pregnancies were observed following clomiphene citrate
used without assisted reproductive technologies. Compared to natural conception, the use of IUI with or without OS
was associated with an increased risk of multiple pregnancy (pooled RR 9.73, 95% CI 7.52 -12.60; p= 0.000; 6 studies).
Compared to natural conception, the use of any OS alone was associated with an increased risk of any MCM (RR pooled
1.18, 95%CI 1.03-1.36; 11 studies), major musculoskeletal malformations (pooled RR 1.48, 95%CI 1.21-1.81; 7 studies),
and malformations of the nervous system (pooled RR 1.73, 95%CI 1.15-2.61; 6 studies). Compared to natural conception,
the use of IUI was associated with an increased risk of any MCM (pooled RR 1.23, 95%CI 1.10-1.37; 10 studies), major
urogenital (pooled RR 1.52, 95%CI 1.04-2.22; 7 studies), and musculoskeletal malformations (pooled RR 1.54, 95%CI
1.20-1.98; 7 studies). The overall quality of the included studies was acceptable.
Conclusions: The increased risk of multiple pregnancy and certain types of MCM associated with the use of less invasive
fertility treatments, such as OS and IUI, found in this review, highlights the importance of the practice framing. Heterogeneity
in OS protocols, the combination with other fertility agents, the limited number of studies and the methodological
quality differences reduce our ability to draw conclusions on specific treatment. More observational studies, assessing the
risk of multiple pregnancy or MCM, as a primary outcome, using standardized methodologies, in larger and better clinically
defined populations are needed.