Multicentre study; observational, longitudinal prospective, case-control

This is a new protocol to evaluate the mode of birth in women with a low-lying placenta. In particular, we intend to propose a vaginal birth in women having a low-lying placenta with a distance between the inferior placental edge and the internal cervical os, called internal-os-distance (IOD) of > 5 mm, as assessed in the late III trimester using transvaginal sonography.

Duration of the study:

• Duration of the study: 54 months
• Duration of enrollment: 42 months
• Duration of follow-up completion of enrolled cases: 6 months
• Duration of data analysis: 6 months

Study Design:

During the II trimester scan, all women presenting a placenta located in the lower uterine segment will undergo evaluation by Transvaginal Sonography (TVS).

If a placenta previa or a low-lying placenta will be confirmed, the woman will be recruited and asked to participate in our study, by signing a written informed consent. In addition women attending the Maternity Triage with vaginal bleeding at < 316/7 weeks of gestation, with a diagnosis of placenta previa or low-lying placenta and not requiring an emergency delivery will be asked to participate and will be recruited after signing the informed consent.

An expert Obstetrician will perform the TVS as scheduled, after inviting the woman to void.

The assessment will include:

• the measurement of the IOD (first caliper on the internal cervical os and second caliper on the inferior placental edge). In case of a marginal sinus, the distance between the internal cervical os and the marginal sinus will also be assessed;
• the cervical length (defined as shortened if ≤25 mm);
• the placental edge thickness, measured within 1 cm from the meeting point between the basal and the chorionic plate. The placental edge will be considered "thick" if > 1cm or if the angle is >45°.

All women with a resolution of a previa or low-lying placenta will be assessed in accordance with the protocol of each participating Maternity Unit, including a scan assessment at 38-39 weeks of gestation or within 28 days from the due date.

Calculation of sample size / power:

Considering that the incidence of previa and low-lying placenta is approximately 2% at the II trimester scan and 0.4% at birth, and assuming a C.I. of 19% in the probability of vaginal birth in women with low-lying placenta/resolved low-lying (> 20 mm), 27 women will be needed for each participating Maternity Unit at the late III trimester scan. Anticipating a 10% drop out, 30 women will be needed to achieve a 95% statistical power to identify a clinically relevant difference in the rate of vaginal birth. This leads to the need of recruiting 150 women at the II trimester scan.

CRF and data management:

All data will be recorded through CRF provided by the promoter center and the database will be based on Microsoft Excel.

A sample of about 20-30 patients for each enrollment center is necessary to have a statistical power of 95% in detecting a clinically relevant difference in outcomes in the various study groups.

Analysis plan:

Descriptive statistics will be performed for all variables evaluated in the study population. Variables will be described by mean and standard deviation if normally distributed, otherwise by median and interquantile range; proportions will be used for categorical variables. The quantitative variables, among the study groups defined by the IOD at the last TVS, will be compared by parametric and non-parametric tests, whereas the categorical variables will be compared using Pearson's chi2 test (Fisher exact test where appropriate). The analyses for the primary outcome measure will be performed among women admitted to labor. A multivariate analysis will be conducted to assess the association between obstetric variables and vaginal birth.

A p-value<0.05 will be considered significant.

Inclusion Criteria:

• Minimum age of 18
• Women with placenta previa or low-lying placenta confirmed by TVS at 19-23 6/7 weeks of gestation.

Women attending Maternity Triage with vaginal bleeding at < 32 weeks of gestation, with a diagnosis of placenta previa or low-lying placenta and not requiring an emergency delivery.

• Women with a normally located placenta at the II trimester ultrasound scan, at 19-23 6/7 week of gestation (control group)
• Single pregnancy
• Signature of the informed consent to participate in the study

Note: Women of the "control group" will be recruited in a 1:3 ratio. After the inclusion of 1 case, 3 women with a normally located placenta will be recruited, according to the parity of the woman representing the case (e.g.: 1 CASE= nulliparous woman, 3 CONTROLS= 3 nulliparous women).

Exclusion Criteria:

• Suspected or confirmed invasive placentation (i.e., placenta accreta)
• Vaginal bleeding requiring emergency delivery
• Inability to meet the conditions set out in the study protocol

Women having a persisten low-lying placenta with an IOD between 6 and 20 mm and women with a resolved previa or low-lying placenta with an IOD > 20 mm who will be offered a trial of labor.

A control group will be represented by women with a normal placental location at the II trimester scan.