Introduction: To date, few studies have evaluated the effects of probiotics in women at high risk of spontaneous preterm birth (PTB), which have presented contradictory results.
Objective: This study aims to investigate the efficacy of administrating probiotics together with 17α-hydroxyprogesterone (17α-OHP) on spontaneous PTB and related pregnancy outcomes in pregnant women at high risk for PTB.
Materials and Methods: This randomized clinical trial was conducted on 118 pregnant women at high risk for PTB (with a history of PTB or pregnancy termination in the second trimester) receiving 17α-OHP injection (250 mg, IM). They were assigned to probiotic group (n=58) and placebo group (n=60). The probiotic group received a 500 mg Lactofem bio-capsules orally and daily, containing Lactobacillus acidophilus 2×109 cfu/g, Bifidobacterium bifidus 2×109 cfu/g, Lactobacillus rutri 2×109 cfu/g, Lactobacillus fermentum 2×109 cfu/g, from the 16th to the 37th week of pregnancy. The placebo capsules contained starch powder. The obstetric outcomes included Preterm Premature Rupture of Membranes (PPROM), PTB and mode of delivery. The neonatal outcomes included anthropometric characteristics and Apgar score (at 1 and 5 minutes after birth). The obtained data were analyzed using t-test and chi-square test. The significance level was set at 0.05.
Results: The mean age of the participants was 30.27±7.56 and 28.93±7.32 years in the probiotic and control groups, respectively. Their mean body mass index (BMI) was 26.80±2.12 and 26.74±2.98 kg/m2, respectively. Also, 8.62% and 15% of women in the probiotic and placebo groups had PTB before the 34th week of pregnancy, while 12.06% and 16.7% had PTB from the 34th to the 37th week of pregnancy, respectively. There were no significant differences between the two groups in these obstetric outcomes. After delivery, the newborn’s weight was 2928.07±454.83 and 2879.16±348.27; head circumstance was 33.39±1.15 and 33.46±1.46; height was 49.58±1.30 and 49.93±1.45; Apgar score at 1 minute after birth was 8.7±0.6 and 8.6±0.5, and Apgar score at 5 minutes after birth was 9.8±0.6 and 8.9±0.8, respectively. There were no significant differences between the two groups in these neonatal outcomes.  
Conclusion: The use of probiotic adjuvant in combination with 17α-OHP injection from the 16th to the 37th week of pregnancy cannot reduce the risk of spontaneous PTB or improve neonatal and obstetric outcomes in women at high risk for PTB.