CELL JOURNAL(Yakhteh), Vol 19, No 3, Oct- Dec (Autumn) 2017 506 Original Article Determinants of The Grade A Embryos in Infertile Women; Zero-Infated Regression Model Amir Almasi-Hashiani, M.Sc., Azadeh Ghaheri, M.Sc., Reza Omani Samani, M.D.* Department of Epidemiology and Reproductive Health, Reproductive Epidemiology Research Center, Royan Institute for Reproductive Biomedicine, ACECR, Tehran, Iran *Corresponding Address: P.O.Box:16635-148, Department of Epidemiology and Reproductive Health, Reproductive Epidemiology Research Center, Royan Institute for Reproductive Biomedicine, ACECR, Tehran, Iran Email: samani@royaninstitute.org Received: 29/May/2016, Accepted: 21/Aug/2016 Abstract Objective: In assisted reproductive technology, it is important to choose high quality em- bryos for embryo transfer. The aim of the present study was to determine the grade A embryo count and factors related to it in infertile women. Materials and Methods: This historical cohort study included 996 infertile women. The main outcome was the number of grade A embryos. Zero-Infated Poisson (ZIP) regres- sion and Zero-Infated Negative Binomial (ZINB) regression were used to model the count data as it contained excessive zeros. Stata software, version 13 (Stata Corp, College Sta- tion, TX, USA) was used for all statistical analyses. Results: After adjusting for potential confounders, results from the ZINB model show that for each unit increase in the number 2 pronuclear (2PN) zygotes, we get an increase of 1.45 times as incidence rate ratio (95% confdence interval (CI): 1.23-1.69, P=0.001) in the expected grade A embryo count number, and for each increase in the cleavage day we get a decrease 0.35 times (95% CI: 0.20-0.61, P=0.001) in expected grade A embryo count. Conclusion: There is a signifcant association between both the number of 2PN zygotes and cleavage day with the number of grade A embryos in both ZINB and ZIP regression models. The estimated coeffcients are more plausible than values found in earlier studies using less relevant models. Keywords: Embryo Research, Assisted Reproductive Technology, Cleavage Stage, Poisson Distribution, Zygote Cell Journal (Yakhteh), Vol 19, No 3, Oct- Dec (Autumn) 2017, Pages: 506- 511 Citation: Almasi-Hashiani A, Ghaheri A, Omani Samani R. Determinants of the grade A embryos in infertile women; zero-infated regression model. Cell J. 2017; 19(3): 506-511. doi: 10.22074/cellj.2017.4214. Introduction On the basis of The International Committee for Monitoring Assisted Reproductive Technology and the World Health Organization (WHO), infertility is defned as failure to achieve a clinical pregnancy with having regular unprotected intercourse for 12 or more months (1, 2). Infertility has been considered as one of the global public health issue in the world by WHO with approximately 80 million infertile couples worldwide (3, 4). Assisted reproductive technology (ART) is a collection of medical steps for the treatment of infertility. From 1978 (when the frst ART baby was born) to 2012, ART has contributed to the birth of more than 5 million infants worldwide (5). In developed countries, approximately 1% of all infants are the product of in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI) treatments (6). In recent decades, success rates in ART have signifcantly improved (7). Although ART may help infertile couples to become pregnant, successful outcomes after ART interventions depend on many related factors (5, 8), such as embryo quality (8). To make sure that infertile women have a good possibility of getting pregnant it is vital to select and transfer embryos of the best quality; particularly when only one embryo is transferred at a time as higher