The aim of this study is to compare the effectiveness between the cervical pessary and the natural progesterone in reduction of preterm birth rates in pregnant women with a uterine cervical length of 25 mm or less evaluated by transvaginal ultrasonography.

Preterm birth is the leading cause of perinatal morbidity and mortality. The rates of spontaneous premature labor have not changed much during the past 10 years. A significant decrease in mortality and morbidity of premature babies will only be possible if women at risk of spontaneous preterm birth are accurately identified and administered preventive therapies. Ultrasonographic measurement of the cervix between 20 and 24 weeks of gestation can improve the identification of both women with single pregnancies and those with twin pregnancies at risk. Asymptomatic women with a short uterine cervix (25 mm or less) are at increased risk of spontaneous premature labor.

The prophylactic use of progesterone during the early phase of pregnancy in women with a history of preterm birth and those with a short cervix can prevent preterm birth.

The cervical pessary is a device used also for the prevention of preterm birth. Vaginal infections are also important causes of preterm birth. There are not many studies about the vaginal microbiome in pregnant women.

Measurement of cervical length is used as a screening test because it is inexpensive, has a short learning curve, and is well tolerated by patients. In addition, placement and removal of the pessary is an easy, accessible, and noninvasive procedure. The results on the concentration of omega 3 and preterm birth are still conflicting.

The aim of this study is to compare the effectiveness of the cervical pessary and the natural progesterone in reduction of preterm birth rates in pregnant women with a uterine cervix measuring 25 mm or less in length as evaluated by transvaginal ultrasonography, assess whether there is a relationship between maternal plasma concentration of omega 3 and preterm birth, and compare the microbiome in these women.

Methods: A prospective randomized controlled trial including pregnant women at the time of morphological ultrasound between 20 and 23 weeks and 6 days of pregnancy. Pregnant women in this gestational age with cervical length of 25 mm or less will be randomized between the conduct and the inclusion of progesterone vaginal pessary. In patients of both groups vaginal discharge sample will be collected at the time of randomization.

Inclusion Criteria:

• gestational age between 20 weeks and 23 weeks and 6 days
• singleton pregnancies

Exclusion Criteria:

• fetal death at randomization
• major structural or chromosomal abnormality
• cervical cerclage in the current pregnancy
• preterm rupture of membranes diagnosed before randomization