Tobacco consumption is associated with slow-growing day-6 blastocysts.

Objective: To investigate if there are any obvious clinical factors associated with delayed blastulation at day 6 (D6) compared with day 5 (D5).

Design: Monocentric observational cohort study from November 2012 to December 2018.

Setting: Tertiary-care academic medical center.

Patients: A total of 941 women with an entire cohort of exclusively D5 blastocysts compared with 162 patients with a cohort of exclusively D6 blastocysts.

Interventions: None.

Main Outcome Measures: Clinical characteristics and data related to the ovarian stimulation protocols.

Results: After univariate analysis, a significantly higher proportion of women who were active smokers was found in the D6 group compared with the D5 group (n = 22/162 [13.6%] vs. n = 82/941 [8.7%]). In addition, the women in the D6 group had a higher rank number of assisted reproductive technology (ART; total no. of ART cycles performed: 2.1 ± 1.4 vs. 1.6 ± 1.1) and a lower antral follicle count (AFC; 18.7 ± 11.3 vs. 22.2 ± 12.8). Moreover, fertilization with the use of intracytoplasmic sperm injection was used more frequently in the D6 group compared with the D5 group. Logistic regression analysis adjusted for confounders highlighted several independent predictors for reaching blastocyst stage at D6 rather than D5: being an active smoker, previous ART cycles, and a lower AFC.

Conclusions: Obtaining an exclusively D6 blastocyst cohort is independently associated with women who are active smokers, previous ART cycles, and a lower AFC. These findings provide evidence, to be confirmed by further studies, that women who are active smokers could greatly benefit from smoking cessation before undergoing ART.