en Gynecology & Obstetrics Gynecology & Obstetrics Original Research Original Research <p style="text-align: left"><strong>Background:</strong> Threatened preterm labor (TPL) is the leading cause of hospitalization during pregnancy. Tocolytic agents are the<br> primary therapeutic options for TPL. The aim of this study is to compare intramuscular progesterone with oral nifedipine as a tocolytic<br> agent.<br> <strong>Methods: </strong>This randomized controlled trial was carried out in a teaching hospital (Shahid Akbarabadi) in Tehran, Iran, from December<br> 2011 to November 2012. Three hundred and fifteen singleton pregnant women aged >18 yrs at 26-34 weeks&rsquo; gestation with the diagnosis<br> of threatened preterm labor (TPL) were randomly received either intramuscular progesterone or oral nifedipine for tocolysis. Maternal<br> and neonatal outcomes were then compared between the two interventions. P value less than 0.05 was considered statistically significant.<br> IRCT registration number of this study is IRCT201112198469N1<br> <strong>Results: </strong>The success rate of progesterone and nifedipine in treating TPL were 83% and 82.7%, respectively. There was no significant<br> difference between the two interventions with regard to gestational age at delivery, type of delivery, the time interval until the delivery,<br> birth weight, NICU admission rate and hospital stays. Progesterone administration was associated with lower duration of NICU stay as<br> compared with nifedipine (0.33&plusmn;0.77 days vs.1.5&plusmn;3.2 days, p<0.05). None of the two drugs caused any major side effects.<br> <strong>Conclusion: </strong>Single dose intramuscular progesterone is as effective as oral nifedipine in treating TPL. It also significantly reduces the<br> NICU stay.</p> Progesterone, Nifedipine, Threatened preterm labor 327 331 http://mjiri.iums.ac.ir/browse.php?a_code=A-10-1741-5&slc_lang=en&sid=1 Ladan Haghighi dr.ladanhaghighi@yahoo.com 200319475328460033990 200319475328460033990 Yes Endometriosis Research Center, Rasoul-e-Akram Hospital. Iran University of Medical Sciences. Tehran. Iran Mandana Rashidi dr.mrashidi@hotmail.com 200319475328460033991 200319475328460033991 No Shahid Akbar-abadi Hospital, Iran University of Medical Sciences, Tehran, Iran. Zahra Najmi zahranaj@yahoo.com 200319475328460033992 200319475328460033992 No Zanjan University of Medical Sciences, Zanjan, Iran. Homa Homam homam.homa@yahoo.com 200319475328460033993 200319475328460033993 No Rasoul-e-Akram Hospital, Iran University of Medical Sciences, Tehran, Iran. Neda Hashemi nedahashemi1363@yahoo.com 200319475328460033994 200319475328460033994 No Rasoul-e-Akram Hospital, Iran University of Medical Sciences, Tehran, Iran. Alireza Mobasseri dr.mobasseri@yahoo.com 200319475328460033995 200319475328460033995 No Rasoul-e-Akram Hospital, Iran University of Medical Sciences, Tehran, Iran. Yousef Moradi Yousefmoradi211@yahoo.com 200319475328460033996 200319475328460033996 No Pars Advanced and Minimally Invasive Medical Manners Research Center, Pars Hospital, Iran University of Medical Sciences, Tehran, Iran.