Harb M., Mohamed A., Adel A. et al published a study comparing vaginal and intramuscular progesterone to prevent premature labor in women with a history of spontaneous preterm labor recurrence.

The clinical trial divided the women with a history of prior spontaneous premature labor into 3 groups as- 

1. vaginal progesterone group  
2. intramuscular progesterone group
3. control group.

There was a significantly higher percentage of NICU admission among the control group (63.3%) comparing with the progesterone groups (25.2%). 

There was a significantly higher percentage of NICU admission among the intramuscular progesterone group (40%) compared to the vaginal progesterone group (11.4%).

Thus they concluded that the use of progesterone prophylactically decreases the rate of premature labor, lowers the frequency of uterine contractions, and enhances the symptoms of preterm labor. Also, in women with risk for PTL, vaginally administered progesterone is equally effective as intramuscular progesterone in preventing PTL, with the vaginal route having a superior impact.

Source- Harb M, Mohamed A, Adel A. Vaginal versus Intramuscular Progesterone in the Prevention of Recurrent Preterm Labour. Al-Azhar International Medical Journal, 2021 -. doi: 10.21608/aimj.2021.65704.1426