Objective: To estimate the value of uterine, fetal cerebral Doppler and oligohydramnios to predict abnormal fetal heart rate tracings in pregnancies at or beyond 41 weeks

This will be a nested case-control study that was conducted at University Hospital Dr. Jose E. Gonzalez after the approval of Institutional ethical committee. Women with low risk late term or postterm pregnancy (at least 41 weeks) with confirm gestational age who will be referred to our Maternal-fetal unit and admitted 1 to 2 days prior to induction of labour according institutional protocol will be recruited and constitute the cohort.

Cases will be patients with abnormal intrapartum cardiotocogram (category III fetal heart rate tracing). For each case, four controls will be matched. The primary outcome will be to obtain odds ratios for the Doppler parameters (middle cerebral artery pulsatility index, mean uterine artery pulsatility index and Middle cerebral artery pulsatility index to mean uterine artery pulsatility index ratio) and amniotic fluid index that would be associated with intrapartum category III fetal heart rate tracing.

Inclusion Criteria:

• Singleton pregnancy
• Gestational age of at least 287 days (41 weeks)
• Low risk pregnancy (defined as the absence of maternal or fetal complications other than postterm pregnancy)

Exclusion Criteria:

• Twin or multiple gestation
• Uncertain gestational age
• Non vertex presentation
• Estimated fetal weight below the 10th percentile for gestational age (Hadlock´s formula)
• Fetal macrosomia (estimated fetal weight ≥ 4 Kilos) diagnosed by clinical measurement (Johnson´s technique) or ultrasound measurement (Hadlock´s formula).
• Fetal abnormalities found on routine anomaly scan.
• Maternal medical disorder (preeclampsia, chronic hypertension, diabetes, lupus, hyperthyroidism)
• Prelabour rupture of membranes
• Antepartum hemorrhage
• Any fetopelvic disproportion that could create dystocia during labor.

Women with low risk late term or postterm pregnancy (at least 41 weeks) with confirm gestational age by first trimester ultrasound examination, who will be referred to our Maternal-fetal unit and admitted 1 to 2 days prior to induction of labour according institutional protocol.