Study question (Words: max 50) Does cervical conisation add an additional risk of preterm delivery in singleton and twin pregnancies after ART? What embryo transfer strategy should be recommended to women with previous cervical conisation? Summary answer (Words: max 50) The prevalence of conisation is higher in ART vs. spontaneously conceved (SC )singleton pregnancies but similar in ART vs. SC twin pregnancies. Conisation adds an additional risk of PTB in ART singletons of 1.55 and in ART twin deliveries conisation doubles the risk of PTB. What is known already (max 75 words) ART conception and cervical conisation both increases the risk of preterm birth. One study has shown that conisation adds an additional risk to the risk of preterm birth in ART pregnancies. This additional risk, however, was mostly related to higher maternal age and more nullipara among women pregnant after ART. Study design, size, duration (Words: max 50) National cohort study from 1995-2009 in Denmark including ART singleton (n=16,923) and twin (n=4829) deliveries identified through the Danish IVF and Medical Birth register (MBR). A random sample of spontaneously conceived (SC) singletons, two-fold the size of the ART singleton group matched by date and year of birth (n=33,835) and all SC twin deliveries born 1997-2009 were extracted via the MBR (n=15,112). Participants/materials, setting, methods (Words: max 50) Information on all women who had a cervical conisation prior to delivery was retrieved from the Danish Registry of Pathology (DRP). Risk of preterm birth was assessed in multiple logistic regression analyses with adjustment for maternal age, parity and year of birth. Risks were presented as adjusted odds ratio (aOR). Main results and the role of chance (Words: max 125) The prevalence of conisation was higher in women with ART singleton deliveries 3.4% compared to the SC singleton group (2.4%) (P< 0.001). In ART and SC twin deliveries the prevalence of mothers with cervical conisation did not differ. In ART singleton deliveries the prevalence of PTB was 13.1% if conisation was performed prior to delivery vs. 8.2% in women without conisation with an adjusted risk of PTB in mothers with conisation of aOR 1.55 (95%CI 1.21-2.06). In ART twin deliveries the prevalence of PTB was 58.0% vs. 41.3% in women with and without conisation with an adjusted risk of (aOR 1.93 (95%CI 1.35-2.76). The risk of PTB in ART vs. SC singletons with previous conisation was aOR 1.58 (1.05-2.36) and in ART vs. SC twins with previous conisation the aOR of preterm birth was aOR 1.34 (95%CI 0.88-2.02). These added risks of ART were similar to those observed in women without conisation. Limitations, reasons for caution (Words: max 50) We were not able to adjust for the depth of the cervical cones, which has recently been shown to the associated with the risk of preterm birth. Only pregnancies with gestational age 22+0 at delivery were included as deliveries with lower gestational age are defined as miscarriages and not included in MBR. Wider implications of the findings (Words: max 75) With nearly 60% of the ART twins being born preterm in case of conisation and with a doubled risk of PTB in case of conisation, single embryo transfer should be performed in all women with conisation prior to ART independent on age and number of ART cycles performed . With more than ten percent being born preterm and with an an added risk of 1.55 of PTB in ART singletons with previous, the antenatal care should be intensified in ART singleton pregnancies with previous conisation. Study funding/competing interest(s) No research grants were obtained for this study. The study was funded from the involved fertility clinics. No conflict of interests to declare. (Words: 23) Trial registration number N/A

Oral præsentation. Forfattere: Pinborg A, Lykkegaard GØ, Loft A, Rasmussen SC, Ingerlev HJ