Preterm birth by vacuum extraction and neonatal outcome: a population-based cohort study

A number of independent variables were collected; the maternal anthropometrics included: age, height, and body mass index (BMI). BMI was calculated from measured height and weight obtained at the first antenatal care visit, which occurred before the 15^th week of gestation in more than 95% of the pregnancies. BMI was categorized into underweight (below 18.5 kg/m²), normal (18.5–24.9 kg/m²), overweight (25–29.9 kg/m²), obese (>29.9 kg/m²), or missing. Parity was categorized as primi- or multiparity. Information on complications during pregnancy and delivery were coded according to the International Classification of Diseases (ICD) Tenth Revision (1997 and onwards). The following pregnancy complications were included: diabetes – both gestational and types 1 and 2 (O24.0-9) preeclampsia- both hypertension, preeclampsia, and eclampsia (O10.0-O15.9). Labor-related risk factors or covariates included epidural analgesia (EA; yes/no), and induction of labor (yes/no). Indications for operative delivery were classified into four major groups: prolonged labor (O62.0-2, O63.0-9), signs of fetal distress (O68.0-O68.1-9), preeclampsia, and a non-occipitoanterior presentation of the fetus (all presentations except occipitoanterior and breech, registered at birth). Gestational age (GA) for preterm infants was divided into three periods according to the World Health Organization: extremely preterm (before 28 weeks), very preterm (28–31 weeks) and moderately preterm (32–36 weeks). Furthermore, we also divided the preterm gestational period according to guidelines on instrumental delivery into either: less than 34 weeks (VE not recommended), and 34–36 weeks (VE may be used). GA was recorded in completed weeks, and was based on routine ultrasound dating performed at 17 to 18 postmenstrual weeks in 97-98% of all pregnant women. Infant birthweight was categorized as less than 1,500 grams, 1,500-2,000 grams, 2,001-2,500 grams, 2,501-3,000 grams, and 3,001-4,000 grams.

Neonatal diagnoses were classified according to the International Classification of Diseases (ICD) Tenth Revision (1997 and onwards), and identified/collected in the SMBR or in the IPR. The following neonatal outcomes (ICD codes) were assessed: Intracranial laceration and hemorrhage due to birth injury (P10), intracranial non-traumatic hemorrhage of fetus and newborn (P52), convulsions of newborn (P90), other disturbances of cerebral status of newborn (P 91), subgaleal hematoma (P12.2), cephalhematoma (P12.0), and brachial plexus injury (P14.0-3). The definitions of outcomes are described in detail in Table 1.

Neonatal diagnoses of intracranial hemorrhages in preterm infants were mainly based on imaging of the brain using ultrasonography; however, some assessments of the brain at term-equivalent age were alternatively performed with CT and/or MRI. Imaging of the brain was performed on clinical indications only in cases born moderately or late preterm, whereas all very preterm infants (born before 32 weeks of gestation) were screened for intracranial lesions, even in asymptomatic infants. A diagnosis of convulsions included infants with clinical signs of convulsions and/or convulsions verified by EEG.

Statistical analysis was performed using proportions and odds ratios (OR) with a 95% confidence interval (CI) for neonatal complications in relation to mode of delivery, using unassisted VD as the reference group (SPSS 20.0 for Windows software package). Three models were used to assess the relationship between the different modes of delivery and the risk for neonatal complications: one crude, and two adjusted (Models 1 and 2). The included covariates have been shown previously to be related to instrumental deliveries, and were related to the outcomes in cross tabulations. In Model 1, we adjusted for the following confounders or covariates: maternal age, height, BMI, and parity, as well as infant year of birth, birthweight and GA. In Model 2, we added the indication for operative delivery and preeclampsia. The year of birth was entered as a continuous variable in accordance with a linear secular trend, and all other variables were entered as categories. Furthermore, a separate logistic regression analysis was performed to investigate severe ICH in relation to mode of delivery. Here, intraventricular hemorrhages grades 1–2 were excluded and the analysis was adjusted for GA only. We also conducted separated analyses on potential relationships between sex and ICH in relation to mode of delivery. Missing data were entered as a separate category in the analyses. The study was approved by the Regional Ethical Review Board in Stockholm, Dnr 2008/1322-31.

Among the 40,764 (54% of all) preterm deliveries included in this study, 2,319 (5.7%) preterm infants were delivered by VE, 5,505 (13.5%) by CS during labor, and 32,940 (80.2%) by VD. The rate of VE deliveries increased gradually with gestational age, Figure 1.

Table 2 shows maternal and perinatal characteristics of the study population in relation to mode of delivery. The VE rate decreased with maternal height and 80% of the women who delivered by VE were primiparae, compared with 48% of those who underwent CS during labor (not in table). More than 45% of the women who delivered by VE had received epidural analgesia during labor compared with 22% of women with VD, and 12% with CS during labor (not in table). Given the association between GA and VE, infants delivered with VE had higher birthweights.

The most common indication for VE was fetal distress (42%), followed by prolonged labor (25%). Having a non-occipitoanterior position (25%) or fetal distress (17%) were the most common indications for CS during labor, while only 3% in this group had a diagnosis of prolonged labor.

The proportion of preterm infants diagnosed with an ICH varied more than hundred-fold in relation to GA. It decreased from 21.5% among preterm infants born at 22–28 weeks of GA to 0.1% among those born after 36 weeks of gestation. The rates of neonatal convulsions among preterm infants decreased from 2.0% at 22–28 weeks to 0.25% among those born after 36 weeks of gestation, Figure 2. The proportion of preterm infants diagnosed with other disturbances of cerebral status (encephalopathy) decreased with GA, while proportion of infants with brachial plexus injuries or ECH increased slightly with GA, Figure 3.

To report outcome in relation to mode of delivery, the study cohort was divided according to the current guidelines on the use of VE as either preterm births occurring between 34–36 weeks of gestation (VE may be used), or those occurring before 34 gestational weeks (VE not recommended). In our cohort, 33,202 (81.4%) of all preterm births occurred at 34–36 gestational weeks, and 7,562 (18.6%) before 34 weeks of GA.

In Table 3, neonatal outcomes before and after 34 + 0 weeks of gestation are presented in relation to mode of delivery. Overall, seven preterm infants were classified as having an ICH due to birth injury, corresponding to a rate of 0.02%; and 612 infants were diagnosed with non-traumatic ICH, corresponding to a rate of 1.5%. Diagnoses of neonatal convulsions and other disturbances of neonatal cerebral status were rare, especially in infants at less than 34 weeks of GA, and occurred more frequently after VE and CS than after VD. Cephalic hematoma was the most frequent complication after VE of preterm infants (n = 72 or 3.1%), occurring much more often after VE than after CS (0.16%) and VD (0.49%). Subgaleal hemorrhage was less frequent, with a total of only18 cases. More than two thirds of those cases occurred in the VE group.

Table 4 shows rates (per 1000), crude and adjusted odds ratios for the outcomes by mode of delivery, and uses infants born by VD as the reference group. After adjustments, preterm infants born by VE had almost doubled OR for ICH, while those born by CS had no increased risk. Preterm infants delivered by VE also had 4.5 times higher OR for extracranial hemorrhages, while those delivered by CS had no increased risk. A total of 540 (88.2%) of the ICH diagnoses consisted of intraventricular bleedings (ICD-codes 52.0-52.3), the type of ICH most frequently seen in preterm infants. Of these, 438 (71.6%) were graded as mild or moderate (grades 1–2). After excluding grades 1–2 of intraventricular hemorrhages from the analyses, the OR for ICH was still significantly increased for infants delivered by VE (OR 2.58, CI 1.27-5.24), but not for those delivered by CS during labor (OR 1.15, CI 0.78-1.70) after adjustment for GA. There was no difference in ICH rates between preterm boys and girls delivered by VE.

The ORs for convulsions were almost doubled in both the VE and CS groups after adjustments for the variables in Model 1. However, further adjustment for indication for operative delivery decreased the odds and made the associations statistically insignificant. Other disturbances of the neonatal cerebral status were significantly increased (two to three times higher) both among infants delivered by VE, and by CS during labor, although the OR was higher in the VE group.

A total of 53 infants were diagnosed with brachial plexus injury. Of these, 14 were born by VE, corresponding to a rate in the VE group of 0.6% and an OR of 6.21 (95% CI: 2.22-17.4) in the fully-adjusted model. In contrast, infants delivered by CS had no increased risk for this complication. Among infants with brachial plexus injury, there were 11 cases of shoulder dystocia (ICD-code O66.0), of which five occurred in the VE group.