Estimation of Fetal Weight by MR Imaging to PREdict Neonatal MACROsomia (PREMACRO Study)

Macrosomia and growth restriction are important causes of perinatal morbidity, at or near to term. However, clear identification of 'at risk' foetuses is difficult and clinical estimates of fetal weight are poor. Historically, ultrasound has been used as a second line in such cases but the accuracy of this imaging modality in the mid- to late third trimester is also limited. Estimated fetal weight (EFW) is an important part of the clinical assessment and is used to guide obstetric interventions, when a fetus is small or large for dates. It frequently is the single most important component guiding interventions, such as induction of labour or Caesarean section. Due to the imprecision of ultrasound-derived EFW, particularly in cases of suspected macrosomia in the 3rd trimester, the investigators believe that these estimates should not be used to make important obstetric decisions regarding mode and timing of delivery and that a more accurate method of assessment could produce better outcomes by restricting interventions to those foetuses at greatest risk. Some publications have already demonstrated that magnetic resonance (MR) imaging derived-EFW close to delivery, is more accurate than ultrasound The goal of the present study is thus to compare the performance of magentic resonance imaging derived-EFW, versus ultrasound derived-EFW at 36 weeks of gestation, regarding the prediction of neonatal macrosomia.

Macrosomia and growth restriction are important causes of perinatal morbidity, at or near to term. However, clear identification of 'at risk' foetuses is difficult and clinical estimates of fetal weight are poor. Historically, ultrasound has been used as a second line in such cases but the accuracy of this imaging modality in the mid- to late third trimester is also limited. Estimated fetal weight (EFW) is an important part of the clinical assessment and is used to guide obstetric interventions, when a fetus is small or large for dates. When a diagnosis of intra-uterine growth restriction (IUGR) is made, the decision-making process is complex, particularly at very early gestations and involves multiple different factors, including maternal status, cardiotocography, liquor volume and dopplers. However, a large body of research is now available to assist with the management of both early and late-onset intrauterine growth restriction (IUGR) but there is a paucity of evidence to guide clinical practice, once macrosomia has been diagnosed, therefore the EFW is frequently the single most important component guiding interventions, such as induction of labour or Caesarean section. Fetal macrosomia is associated with a higher incidence of perinatal morbidity, including shoulder dystocia and brachial plexus injury in the fetus and anal sphincter tears, uterine atony and haemorrhage in the mother. A recent multicentre randomised controlled trial appears to confirm the advantages of a policy of induction of labour for suspected macrosomia, demonstrating a clear reduction in the rates of shoulder dystocia and composite perinatal morbidity. However, some earlier but lower quality, observational studies have questioned the benefit of EFW made by ultrasonography in the last trimester, for suspected macrosomia, demonstrating that this practice can increase the risk of caesarean and instrumental delivery, without reducing perinatal morbidity. Despite this conflicting data and a lack of evidence to support routine third trimester ultrasound, the absence of specific guidance, coupled with concerns regarding perinatal outcomes,mean that obstetricians will increasingly request an ultrasound at around 34-36 weeks gestation to identify foetuses above the 90th or below the 10th centiles. This practice will inevitably lead to increased and potentially harmful interventions based on relatively inaccurate data. Due to the imprecision of ultrasound-derived EFW, particularly in cases of suspected macrosomia in the 3rd trimester, the investigators believe that these estimates should not be used to make important obstetric decisions regarding mode and timing of delivery and that a more accurate method of assessment could produce better outcomes by restricting interventions to those foetuses at greatest risk. Some publications have already demonstrated that magnetic resonance (MR) imaging derived-EFW close to delivery, is more accurate than ultrasound, with a mean percentage error superior to that of ultrasound and a recent meta-analyses has confirmed this promising accuracy. The goal of the present study is thus to compare the performance of magentic resonance imaging derived-EFW, versus ultrasound derived-EFW at 36 weeks of gestation, regarding the prediction of neonatal macrosomia.

Macrosomia, Magnetic Resonance- Estimated Fetal Weight, Ultrasound-Estimated Fetal Weight, Large-for-date fetus

During routine third trimester ultrasound scan between 30 weeks and 35 weeks +6 days of gestational age, all women with an apparently normal, live singleton pregnancy, planning to deliver at the investigator's hospital maternity, will be invited to participate in the study. An Ultrasound scan to estimate the fetal weigth will be carried out during the 36th week of gestation.

During routine third trimester ultrasound scan between 30 weeks and 35 weeks +6 days of gestational age, all women with an apparently normal, live singleton pregnancy, planning to deliver at the investigator's hospital maternity, will be invited to participate in the study. A Magnetic Resonance examination to estimate the fetal weigth will be carried out during the 36th week of gestation.

Prenatal Ultrasound examinations will be carried out using transabdominal sonography only by experienced consultants in MFM. Ultrasound-Estimated Fetal Weight will be obtained between 36.0-36.6 weeks of gestation, according to Hadlock et al. Operators performing the Ultrasound-Estimated Fetal Weight will be blinded to the results of Magnetic Resonance-Estimated Fetal Weight. The participants, general practitioners, obstetricians and midwifes of the patients will be aware of the results of Ultrasound-Estimated Fetal Weight which will be used for clinical management. For the primary outcome measure, macrosomia during Ultrasound-Estimated Fetal Weight will be defined as ≥ P95 based on Yudkin et al. For secondary outcome measures, it will be redefined as ≥ P90 or ≥ P99 based on Yudkin.

MRI will be performed the same day as the Ultrasound examination, using a clinical 1.5T whole-body unit. Operators performing Fetal Body Volume measurements will be blinded from Ultrasound-Estimated Fetal Weight results. Magnetic Resonance-Estimated Fetal Weight will be calculated using the equation 0,12+1,031*Fetal Body Volume = MR imaging weight (g) developed by Baker. General practitioners, obstetricians and midwifes of the patients will be blinded to the results of the Magnetic Resonance-Estimated Fetal Weight. For the primary outcome measure, macrosomia will be defined as ≥ P95 based on Yudkin et al. For secondary outcome measures, it will be defined as ≥ P90 or ≥ P99.

AUROC for prediction of macrosomia (≥ P95 for gestational age; normal ranges of Yudkin et al.) with MR (4 mm ST (slice thickness)/ 20 mm gap) versus US using the Hadlock equation.

AUROC for prediction of macrosomia (≥ P90 for gestational age) with magnetic resonnance (4 mm slice thickness/20 mm gap) versus ultrasound.

AUROC for prediction of macrosomia (≥ P99 for gestational age) with Magnetic Resonance (4 mm slice thickness/ 20 mm gap) versus Ultrasound.

AUROC for prediction of macrosomia (≥ P97 for gestational age) with Magnetic Resonance (4 mm slice thickness/ 20 mm gap) versus Ultrasound.

Area Under the Receiver Operating Curve (AUROC) for prediction of macrosomia (Abdominal Circumference)

AUROC for prediction of macrosomia with Abdominal Circumference ≥ P90 for gestational age. Measured in cm with Ultrasound

Ability of Magnetic Resonnace-Estimated Fetal Weight (+/- pelvimetric measurements) vs. Ultrasound-Estimated Fetal Weigth in predicting significant shoulder dystocia. Significant shoulder dystocia is defined clinically as difficulty with delivery of the shoulders that was not resolved by the McRoberts' manoeuvre (flexion of the maternal thighs), usually combined with suprapubic pressure. Manoeuvres whose use suggested significant shoulder dystocia were those involving rotation of the fetus to displace the anterior shoulder impacted behind the maternal pubic bone (Woods, Rubin, or Jacquemier manoeuvres). The definition also included births with an interval of 60 s or more between delivery of the head and the body.

Ability of Magnetic Resonance-Estimated Fetal Weigth (+/- pelvimetric measurements) vs. Ultrasound-Estimated Fetal Weigth in predicting maternal morbidity, defined as caesarean section, operative vaginal delivery (vacuum or forceps), postpartum haemorrhage (1000 mL or more), blood transfusion, and anal sphincter tear.

Ability of Magentic Resonance-Estimated Fetal Weigth (+/- pelvimetric measurements) vs. Ultrasound-Estimated Fetal Weigth in predicting neonatal morbidity, defined as arterial cord blood pH less than 7.10, Apgar score at 5 min less than 7, and admission to the neonatal intensive-care unit.

Ability of Magentic Resonance-Estimated Fetal Weigth (+/- pelvimetric measurements) vs. Ultrasound-Estimated Fetal Weigth in predicting neonatal hyperbilirubinaemia, defined as a maximum value exceeding 350 mmol/L of blood bilirubin.

Inclusion Criteria: Subjects is ≥ 18 years of age and able to provide a written informed consent. Subject is a pregnant woman carrying a live singleton fetus at the 36+0-36+6 weeks scan, with no major abnormalities appearing during prenatal imaging with no major abnormalities appearing during prenatal imaging potentially affecting the correct use of the Hadlock formula for US-EFW. Conditions such as congenital diaphragmatic hernia with decreased abdominal circumference could be underestimated by the Hadlock USEFW. Another example is a massive sacro-coccygial teratomas. Subject is planning a delivery at our maternity at the University Hospital Brugmann, in Brussels, Belgium. Subject is known not to have any contra-indication to undergo an MR imaging examination. Exclusion Criteria: Subject is known to have a contra-indication to undergo an MR imaging examination such as: Carrying a pacemaker or a metallic cardiac valve, having metallic material inside the head, having metallic fragments inside the eye following an accident, having any type of implant including ear implant, having a hip prosthesis Subject presenting with painful regular uterine contractions or history of ruptured membranes. Subjects who are unconscious, severely ill, mentally handicapped or under the age of 18 years. If birth occurs before MR and US evaluation. If patients delivers outside our local maternity unit. If the neonate's weigh is not measured within 6 hours after birth for any reason, including the need for emergency care immediately after delivery

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