Objective
We estimated the risk of complications at birth of extremely large babies (≥5000 g).
Study Design
This was a cohort study including all births of extremely large babies in 1996 through 2005 and comparison cohort with normal birthweight (1:2) identified in the national birth registration.
Results
There were 343 extremely large babies or 0.9% of all singletons. Compared to the normal birthweight cohort (n = 679), there were increased odds of shoulder dystocia (odds ratio [OR], 26.9; 95% confidence interval [CI], 11.1–65.1), emergency cesarean section (OR, 5.2; 95% CI, 3.4–8.0), and failed labor induction (OR, 4.3; 95% CI, 1.7–11.0). The risk of elective section was not increased (OR, 1.1; 95% CI, 0.6–2.0). Minor congenital malformations were more frequent (OR, 2.1; 95% CI, 1.2–3.7), as were birth injuries (OR, 3.7; 95% CI, 2.1–6.8) and minor metabolic disturbance (OR, 2.5; 95% CI, 1.1–6.2), but not asphyxial births.
Conclusion
The risk of shoulder dystocia for very large babies is markedly raised, as are minor complications, while for mothers the main risk is emergency section.
An infant weighing ≥4500 g at birth is generally thought of as macrosomic. Birthweight in Iceland is high and the prevalence of such babies in this well-nourished population is considerably higher than in the United States (7.0% vs 1.4% in 2001) or Europe and has been increasing over the last decades. Macrosomic babies have a higher risk of neonatal death, stillbirth, birth injuries, and various neonatal complications such as sudden hypoglycemia and respiratory problems. Their mothers more often have induced labor, face a higher chance of cesarean section, and are more likely to sustain injuries like serious pelvic floor damage with resulting urinary and/or fecal incontinence, hemorrhage at or after delivery, and prolonged recovery after the birth.
Known risk factors for fetal macrosomia include maternal obesity and preexisting or gestational diabetes, while both maternal and paternal stature also affect birthweight through genetic mechanisms. Mild glucose and lipid metabolic disturbances are of importance along with excessive gestational weight gain. Obesity is an independent risk factor for various birth complications and the risk increases with a higher stage of obesity.
A birthweight of ≥5000 g has been defined as extreme macrosomia. The risk of complications for the mother and the macrosomic infant increases with rising birthweight, as seen from the doubled chance for prolonged neonatal unit care at and above this weight. A previous study on 111 births of extremely macrosomic infants showed that only 44% had a vaginal delivery and in those instances a fifth were complicated by shoulder dystocia and 6% developed Erb’s palsy. A smaller case-control study with 47 cases showed similar results. However, most studies on macrosomic infants have considered babies with birthweight >4000 g or >4500 g. Much less is known about extreme macrosomia, even though the risk in these deliveries can be expected to be highest. When such babies have been suspected before birth it is not straightforward to choose the appropriate time and mode of delivery. As the mean birthweight in Iceland is among the highest in the world, we have undertaken a cohort study to explore specific risk factors associated with this situation.
Materials and Methods
This was a retrospective, register-based cohort study including all singleton babies with birthweight ≥5000 g in the whole population of Iceland (mean population size during the study period: 280,752) from 1996 through 2005 inclusive (exposed cohort). A comparison cohort (ratio 1:2) with birthweight between the 10th-90th percentile for the Icelandic population was selected and individually matched with exposed births for calendar time (date), ie, the comparison cohort babies were born immediately before and after the large babies, of the same gestational age (±14 days), sex, and parity (nulliparity and multiparity). If these conditions were fulfilled, the comparison births were matched as closely as possible for place of birth.
The exposed cohort was identified from the computerized files of the Icelandic Birth Registration and verified by inspection of the individual birth notification forms. Each set of the relevant notes for all mothers, including the neonatal record and discharge summaries, was inspected by one of the authors (H.V.) and verified as appropriate (R.T.G. and H.H. for maternal, A.D. for neonatal data). The International Classification of Diseases, Ninth Revision system was used up and through 1996 and International Statistical Classification of Diseases, 10th Revision (ICD-10) thereafter. Additional population information was obtained from the Statistics Iceland.
Maternal variables were height and weight at midpregnancy (as near to 20 weeks’ gestation as possible), gestational weight gain, estimated birthweight (clinical and ultrasound), maternal age at birth (≤20, 21-25, 26-30, 31-35, and ≥36 years), smoking, and complications during the pregnancy or labor (including trauma). Body mass index (BMI) at midpregnancy was categorized into 4 groups (<19, 19-24, 25-30, >30). BMI was also estimated at the end of pregnancy as was BMI increase. Mean symphysis-fundal height and mean inpatient stay after delivery were recorded. Maternal complications were categorized into 10 groups: infections, gestational hypertension, musculoskeletal complications and exhaustion in pregnancy, gestational and preexisting diabetes mellitus, other pregnancy-related disease, complications during delivery (not elsewhere specified), instrumental delivery, pelvic floor trauma, elective or emergency cesarean section (defined as section at <8 hours from decision). We also recorded the beginning of labor (spontaneous, induction, failed induction).
Neonatal variables included weight, length and head circumference, gestational age, complications during delivery (including trauma), and ponderal index. Neonatal diagnoses were categorized into 9 groups: infections, congenital malformations, birth trauma, shoulder dystocia (defined by ICD-10 diagnosis, verified by scrutiny of the delivery situation by 2 authors), asphyxia, breathing difficulties, metabolic disturbances including hypoglycemia, hematologic complications, and other.
The National Bioethics Committee, relevant hospital authorities, and the Icelandic Data Protection Authority approved the study.
Statistical analysis
The Statistical Package for the Social Sciences (SPSS for Windows rel. 12.0.1.; SPSS Inc, Chicago, IL) was used for online recording of the data and statistical analyses. Descriptive analysis and multivariable regression was used to calculate odds ratios (ORs) with 95% confidence intervals (CIs). Paired samples t test were performed, with significance set at a level of P < .05 if multivariable regression was not appropriate. All data were processed in SPSS except for BMI and ponderal index, which were calculated using Microsoft Excel software (Microsoft Corp, Redmond, WA). The logarithm of unnormalized variables was also calculated in Microsoft Excel for analyses in SPSS.
Results
There were 41,377 deliveries, including 40,319 singleton babies, of whom there were 343 extremely macrosomic, all liveborn and constituting 0.9% of singletons and 0.8% of all newborns. Matched deliveries of children of normal birthweight were 686. Sixteen deliveries were excluded, ie, 10 where the mother had delivered a second baby during the study period (8 exposed and 2 nonexposed deliveries), and case records on 6 deliveries were not found (1 exposed, 5 nonexposed). The total mother-infant pairs analyzed were 1013, ie, 334 exposed and 679 nonexposed births.
Maternal characteristics in exposed and nonexposed births are compared in Table 1 . Emergency cesarean section occurred more often among exposed mothers, while vaginal deliveries were fewer. There was no difference between exposed and nonexposed in elective cesarean section rates.
| Macrosomic deliveries (n = 334) | Normal birthweight deliveries (n = 679) | ||||
|---|---|---|---|---|---|
| Characteristic | n | % | n | % | OR (95% CI) a |
| Vaginal delivery | 231 | 69.2 | 607 | 89.4 | 0.3 (0.2–0.4) |
| Elective cesarean section | 21 | 6.3 | 32 | 4.7 | 1.4 (0.8–2.4) |
| Emergency cesarean section | 82 | 24.6 | 40 | 5.9 | 5.2 (3.5–7.8) |
| BMI <19 | 1 | 0.3 | 8 | 1.2 | 0.3 (0.0–2.0) |
| BMI 19-24 | 76 | 22.8 | 296 | 43.6 | 0.4 (0.3–0.5) |
| BMI 25-30 | 134 | 40.1 | 240 | 35.3 | 1.2 (0.9–1.6) |
| BMI >30 | 100 | 29.9 | 112 | 16.5 | 2.2 (1.6–2.9) |
| Nonsmokers | 298 | 89.2 | 537 | 79.1 | 2.2 (1.5–3.2) |
| Mothers who smoked sometime during pregnancy | 35 | 10.5 | 142 | 20.9 | 0.4 (0.3–0.7) |
| Age ≤20 y | 19 | 5.7 | 34 | 5.0 | 1.1 (0.6–2.0) |
| Age 21-25 y | 54 | 16.2 | 120 | 17.7 | 0.9 (0.6–1.3) |
| Age 26-30 y | 118 | 35.3 | 223 | 32.8 | 1.1 (0.8–1.5) |
| Age 31-35 y | 107 | 32.0 | 190 | 29.0 | 1.2 (0.9–1.6) |
| Age ≥36 y | 36 | 10.8 | 112 | 16.5 | 0.6 (0.4–0.9) |
| Gestational diabetes mellitus | 8 | 2.4 | 9 | 1.3 | 1.8 (0.7–4.8) |
a Adjusted for sex, gestational age (±14 d), parity (nulliparity and multiparity).
The mothers of extremely macrosomic babies were more likely to be categorized with a higher BMI than controls ( Table 1 ). The mean BMI was 28.6 (±5.14 SD) among exposed mothers and 26.1 (±4.40 SD) among nonexposed mothers and increased more over the gestation among the exposed mothers (4.2 ± 1.57 SD vs 3.5 ± 1.43 SD). Exposed mothers were less likely to be smokers. Age distribution was similar between exposed and nonexposed mothers, with the only difference seen for the oldest age group, where exposed mothers were less likely to be included. There were no differences with regard to gestational diabetes.
The mean length of labor of exposed births was 435 minutes (minimum-maximum: 62-2030 minutes), but 449 minutes (minimum-maximum: 40-2305 minutes) for nonexposed births. There were no differences in the occurrence of prolonged first (exposed 20.7%; nonexposed 31.2%; OR, 0.8; 95% CI, 0.6–1.1) or second (exposed 1.5%; nonexposed 2.7%; OR, 0.7; 95% CI, 0.2–1.9) stage of labor, but the length of the second stage was longer (mean 30 minutes; minimum-maximum: 3-188 minutes) for exposed mothers compared to nonexposed (21 minutes; minimum-maximum: 0-315 minutes) ( P = .03). Deliveries among exposed mothers were less often spontaneous than among the nonexposed ( Table 2 ). Induction of labor was more likely to fail among exposed mothers with resulting higher rates of emergency cesarean section.
| Macrosomic deliveries (n = 334) | Normal birthweight deliveries (n = 679) | ||||
|---|---|---|---|---|---|
| Characteristic | n | % | n | % | OR (95% CI) a |
| Spontaneous labor | 216 | 64.7 | 560 | 82.5 | 0.4 (0.3–0.5) |
| Induction of labor | 84 | 25.1 | 77 | 11.3 | 2.5 (1.8–3.6) |
| Failed induction of labor, cesarean section | 14 | 4.2 | 7 | 1.0 | 4.3 (1.7–11.0) |
| Elective cesarean section | 20 | 6.0 | 35 | 5.2 | 1.1 (0.6–2.0) |
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