Objective
To estimate the association between placenta previa and abnormal fetal growth.
Study Design
Retrospective cohort study of consecutive women undergoing ultrasound between 15 and 22 weeks. Groups were defined by the presence or absence of complete or partial placenta previa. The primary outcome was intrauterine growth restriction (IUGR), defined as a birthweight <10th percentile by the Alexander growth standard. Univariable, stratified, and multivariable analyses were used to estimate the effect of placenta previa on fetal growth restriction.
Results
Of 59,149 women, 724 (1.2%) were diagnosed with a complete or partial previa. After adjusting for significant confounding factors (black race, gestational diabetes, preeclampsia, and single umbilical artery), the risk of intrauterine growth restriction remained similar (adjusted odds ratio, 1.1; 95% confidence interval, 0.9–1.5). The presence of bleeding did not impact the risk of growth restriction.
Conclusion
Placenta previa is not associated with fetal growth restriction. Serial growth ultrasounds are not indicated in patients with placenta previa.
Placenta previa, defined as a placenta that implants at or over the cervical os, occurs in approximately 0.3-0.5% of pregnancies at delivery. Because of the possibility of maternal hemorrhage, it is a significant contributor to maternal morbidity, as well as prematurity and perinatal mortality. In addition, this inherently abnormal placentation creates concern for fetal well-being and fetal growth. Several features of placenta previa suggest that this patient population is at higher risk of intrauterine growth restriction (IUGR), a significant risk factor for perinatal mortality. First, the blood supply to the lower uterine segment is less than at the fundus, presumably resulting in less perfusion for a placenta previa. Also, repeated bleeding episodes from placental previa may impact fetal oxygenation and growth. Prior studies assessing the association between placenta previa and fetal growth restriction have yielded conflicting results.
Although the majority of placenta previa diagnosed at routine midtrimester anatomy ultrasound will resolve, prediction of previa resolution or persistence is impossible at this time. It is only a late-gestation ultrasound that can definitively establish persistence or resolution of placenta previa, which does little to assist the clinical management of fetal surveillance if a relationship between placenta previa and restricted fetal growth exists. In the many interim weeks, concern for poor fetal growth potential would necessitate additional ultrasounds for fetal growth assessments, as opposed to a single scan in the late third trimester to assess placental location to determine mode of delivery.
We therefore sought to estimate the relationship between placenta previa and fetal growth restriction in an effort to assist physicians with clinical management.
Materials and Methods
We performed a retrospective cohort study of all consecutive patients undergoing routine second-trimester (15-22 weeks) ultrasound at a single tertiary center. Institutional review board approval was obtained. Data were prospectively collected over an 18-year period (1990-2008) by dedicated research nurses. Each patient seen in our center was given a standardized form requesting information regarding the pregnancy outcome, to be returned after delivery. If a form was not returned within 4 weeks of the expected delivery date, the coordinator called the patient. If the patient could not be contacted, the coordinator contacted the referring physician to obtain outcome data. The follow-up form contains details regarding pregnancy complications, delivery complications, and neonatal outcomes.
Patients were included in this study if they had a confirmed singleton gestation delivered after 20 weeks’ gestation. They were excluded if they had a fetal demise at the time of presentation for anatomic survey or if they carried a higher-order fetal gestation. Because women with known major fetal anomalies have an increased risk for growth restriction, they were excluded from this analysis. Gestational age was determined by either last menstrual period if known and concordant with ultrasound (within 7 days of first-trimester ultrasound or 14 days of second-trimester ultrasound) or by the earliest ultrasound available when the last menstrual period was unknown or discordant with ultrasound.
At our institution, second-trimester ultrasounds routinely involve assessment of placenta location. Suspected placenta previa by transabdominal scanning is confirmed with transvaginal ultrasound. Placenta previas diagnosed by transvaginal ultrasound were coded as complete (placenta covering the entire cervical os), partial (placenta covering part of the internal cervical os), or marginal (placenta within 2 cm of the internal cervical os). Comparisons were made between women with no placenta previa and those diagnosed with either a complete or partial previa. As the majority of marginal previas are known to resolve, they were not included in the primary analysis.
The primary outcome was IUGR, defined as a birthweight less than the 10th percentile using the Alexander growth standard for gestational week at delivery. The secondary outcome examined was birthweight less than the fifth percentile using the Alexander growth standard. Secondary analyses included assessing the impact of previa type on fetal growth, as well as examining the effect of previa persistence on fetal growth in the subset of patients who underwent at least 1 repeat ultrasound.
The incidence of placenta previa by previa type was described, and their association with IUGR was estimated. Patients with and without placenta previa were compared with descriptive and bivariate statistics using unpaired Student t tests for continuous variables and χ 2 tests for categorical variables. Potentially confounding variables of the exposure-outcome association were identified in the stratified analyses. Multivariable logistic regression models for IUGR less than the 10th and fifth percentiles were then developed to better estimate the effect of placenta previa on fetal growth while adjusting for potentially confounding effects. Clinically relevant covariates for initial inclusion in multivariable statistical models were selected using results of the stratified analyses, and factors were removed in a backward stepwise fashion, based on significant changes in the exposure adjusted odds ratio (AOR) or significant differences between hierarchical models using likelihood ratio test. In patients with a repeat ultrasound, the rate of previa at the first and the last ultrasound were described. McNemar χ 2 analysis was used to compare the rate of resolution between types of previa, and Pearson χ 2 test for trend across groups. The statistical analysis was performed using STATA, version 10 Special Edition (StataCorp, College Station, TX).
Results
Of 72,373 women who underwent routine second-trimester ultrasound at our facility, complete outcome data were available for 65,414 (90.4%). Within the cohort of 65,414 women for whom complete outcomes data were available, 57,739 remained after exclusions for presence of any fetal anomaly. Of these women, 1665 received a diagnosis of any type of previa on routine second-trimester ultrasound: 392 complete, 332 partial, and 941 marginal. The 724 patients who received a diagnosis of either complete or partial previa were included in the primary analysis. Patients with and without placenta previa differed slightly ( Table 1 ). Patients with placenta previa tended to be slightly older, had more pregnancies, had less preeclampsia, and had higher rates of preterm premature rupture of membranes, bleeding during pregnancy, and intrauterine fetal demise. The 2 groups were similar with respect to tobacco use, diabetes, and single umbilical artery.
| Demographic | No previa (n = 57,015) | Previa (n = 724) | P |
|---|---|---|---|
| Age, y | 30.2 ± 6.3 | 32.5 ± 5.8 | < .01 |
| Gravidity | 2.7 ± 1.6 | 3.1 ± 1.7 | < .01 |
| Parity | 1.1 ± 1.2 | 1.2 ± 1.2 | < .01 |
| Race | |||
| White | 36,337 (63.7%) | 479 (66.2%) | .18 |
| Black | 11,716 (20.5%) | 124 (17.1%) | .02 |
| Tobacco use | 6308 (11.1%) | 94 (13.0%) | .11 |
| Diabetes | 2998 (5.4%) | 38 (5.3%) | .95 |
| Preeclampsia | 4244 (7.6%) | 38 (5.3%) | .02 |
| Preterm premature rupture of membranes | 1311 (2.3%) | 27 (3.8%) | .01 |
| Single umbilical artery | 260 (0.5%) | 2 (0.3%) | .47 |
| Intrauterine fetal demise | 415 (0.7%) | 13 (1.8%) | < .01 |
| Bleeding (any time) | 3286 (5.8%) | 123 (17.0%) | < .01 |
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