Objective
The objective of the study was to evaluate the association between a sonographically diagnosed subchorionic hematoma (SCH) in the first trimester and subsequent midtrimester cervical length and preterm birth.
Study Design
In this cohort study, 512 women with an SCH on their first-trimester ultrasound were compared with 1024 women without a first-trimester SCH. All women underwent routine transvaginal cervical length measurement between 18 and 22 weeks. Women with multifetal gestation, cerclage, or a uterine anomaly were excluded. A multivariable linear regression was performed to assess the independent association of SCH with cervical length, and a logistic regression was done to determine whether the presence of SCH was associated with preterm birth independent of the cervical length.
Results
In a univariable analysis, the presence of a SCH was significantly associated with a shorter mean cervical length as well as a cervical length less than the 10th percentile (4.27 cm vs 4.36 cm, P = .038; 1.9% vs 0.5%, P = .006, respectively). Preterm birth also was more common in women with an SCH (12.5% vs 7.3%, P = .001). Even after adjusting for potentially confounding factors, a significant negative association existed between the presence of an SCH and cervical length (centimeters) (linear regression coefficient, –0.08; 95% confidence interval, –0.17 to –0.005). In a multivariable regression, SCH remained associated with preterm birth, even with cervical length entered into the equation as a covariate (adjusted odds ratio, 1.58; 95% confidence interval, 1.09–2.32).
Conclusion
First-trimester SCH is associated with both a shorter cervical length and preterm birth. Our data suggest, however, that mechanisms other than cervical shortening may be involved in preterm birth among women with SCH.
The reported incidence of first-trimester subchorionic hematoma (SCH) in studies published over the last 3 decades ranges between 1.3% and 40%. The presence of SCH has been associated with an increased risk of adverse outcomes, including miscarriage, intrauterine growth restriction, placental abruption, preeclampsia, and preterm delivery. Although it is possible that these associations are related to an underlying placental dysfunction, the mechanism behind the increased risk of preterm birth remains uncertain.
The finding of a short cervix on transvaginal ultrasound also is a known risk factor for preterm birth. A recent study found that the risk of preterm birth associated with a given midtrimester sonographic cervical length is greater with a history of first- or second-trimester vaginal bleeding. However, the data regarding the association of SCH (with or without vaginal bleeding) and cervical length, and their joint association with preterm birth, are lacking.
Therefore, our objective was to evaluate the association between SCH and sonographically diagnosed short cervix at the midtrimester ultrasound and to assess whether the combination of SCH and a short cervix increases the risk of preterm birth more than a short cervix alone.
Materials and Methods
This was a retrospective cohort study of women undergoing both first-trimester ultrasound and routine midtrimester cervical length assessment, between January 2010 and March 2014, at a single tertiary care institution. Women were included in this cohort if they were at least 18 years of age, had a singleton nonanomalous gestation, and delivered at the same institution where the first-trimester ultrasound and cervical length ultrasound were performed. The presence of SCH on that ultrasound was defined as a retroplacental, hypoechoic region as determined by an obstetric sonologist. Transvaginal cervical length was assessed between 18 0/7 and 22 6/7 weeks at the time of the fetal anatomic survey as a part of routine clinical care in all women. The cervical length was measured in accordance with the methods previously described by Iams et al.
Women’s characteristics and pregnancy outcomes were compared between those with and without the diagnosis of SCH on the first-trimester ultrasound. Demographic and baseline clinical data, such as maternal age, race/ethnicity, body mass index, and prior obstetric history were abstracted from the clinical records. Factors suggested by prior studies to be associated with preterm birth such as prior cervical excisional procedure, prior dilation and curettage, history of sexually transmitted infection, tobacco use, treatment with progesterone, and the presence of vaginal bleeding during the first or second trimester were abstracted as well.
The primary outcome of this analysis was the midtrimester cervical length. It was examined as a continuous measurement as well as a categorical measurement of less than 3.0 cm (ie, the 10th percentile at this gestational age ). Similarly, gestational age at the time of delivery was examined as a continuous outcome variable and a categorical variable (ie, preterm birth <37 weeks). In addition, we examined obstetric outcomes that have been reported to be associated with SCH, including preterm premature rupture of membranes (PPROM), placental abruption, preeclampsia, and small for gestational age (SGA), which was defined as a birthweight less than the 10th percentile based on the Alexander growth standard.
All analyses were performed with Stata version 12.0 (StataCorp, College Station, TX). All tests were 2 tailed and the value of P < .05 was used to define significance. An a priori power analysis was performed.
Assuming women without a SCH at our institution have a 2% frequency of short cervix (<3.0 cm), at an α of 0.05 and a power of 0.80, 512 women with SCH and 1024 without SCH were needed to detect a frequency of short cervix of 5% among women with SCH. Thus, during the period of study, consecutive women with SCH were identified until the required number of cases was reached. Controls were randomly selected from among all women without an SCH during the same period of study.
Univariable comparisons were performed using a Pearson’s χ 2 test for categorical data and the Student t test or Mann-Whitney U for continuous measures. To determine whether there was any interaction between SCH and cervical length on the risk of preterm birth, an interaction term of SCH and cervical length was created. A multivariable linear regression was done to assess the association between the SCH and cervical length. A multivariable logistic regression was used to estimate whether the presence of SCH was associated with preterm birth independent of the presence of short cervix. Covariates entered into the regressions were those that in univariable analysis had an association with SCH at a level of P < .05.
Approval for this study was obtained from the Northwestern University Institutional Review Board.
Results
Five hundred twelve women with SCH were identified and compared with 1024 women without SCH. Maternal characteristics and cervical length measurements, stratified by the presence of SCH are shown in Table 1 . Women with SCH were less likely to be white or nulliparious, had higher rates of prior dilation and curettage and prior cesarean delivery, and have received progesterone treatment. Women with a first-trimester SCH also developed a shorter mean cervical length in the second trimester (4.27 ± 0.76 cm vs 4.36 ± 0.74 cm; P = .038) and were more likely to have a cervical length less than the 10th percentile ( Table 1 ).
| Characteristic | SCH (n = 512) | Control (n = 1024) | P value |
|---|---|---|---|
| Maternal age, y | 34.6 ± 5.0 | 34.4 ± 4.1 | .34 |
| Nulliparous | 149 (29.1) | 384 (37.5) | .001 |
| Body mass index, kg/m 2 | 35.8 ± 5.9 | 29.4 ± 4.9 | .12 |
| Race/ethnicity | .001 | ||
| African American | 71 (13.9) | 101 (9.9) | |
| Non-Hispanic white | 320 (62.5) | 750 (73.2) | |
| Hispanic | 56 (10.9) | 80 (7.8) | |
| Other | 62 (12.1) | 93 (9.1) | |
| Prior dilation and curettage | 111 (21.7) | 178 (17.4) | .04 |
| Prior loop electrosurgical excision procedure or cold-knife cone | 35 (6.8) | 83 (8.1) | .38 |
| Prior cesarean delivery | 81 (15.8) | 111 (10.8) | .005 |
| History of sexually transmitted infection | 51 (10.0) | 90 (8.8) | .45 |
| Current smoker | 14 (2.7) | 35 (3.4) | .48 |
| History of spontaneous abortion | 124 (24.2) | 214 (20.9) | .14 |
| Prior preterm delivery | 25 (4.9) | 60 (5.9) | .43 |
| First- or second-trimester bleeding | 34 (6.6) | 16 (1.6) | < .001 |
| Gestational or pregestational diabetes | 37 (7.2) | 68 (6.6) | .67 |
| Chronic hypertension | 15 (2.9) | 42 (4.1) | .25 |
| Received 17OHP-C or vaginal progesterone this pregnancy | 3 (0.6) | 19 (1.8) | .048 |
Maternal and neonatal outcomes are depicted in Table 2 . Women with a SCH had a higher frequency of preterm delivery, PPROM, and placental abruption. The frequencies of SGA and preeclampsia were similar between the groups. Of 15 women diagnosed with a cervical length less than 3 cm, 3 had cervical length of 2 cm or less, of whom 2 received vaginal progesterone.
| Variable | SCH (n = 512) | Control (n = 1024) | P value |
|---|---|---|---|
| Cervical length at 18–22 wks, cm | 4.27 ± 0.76 | 4.36 ± 0.74 | .038 |
| Cervical length <3 cm | 10 (1.9) | 5 (0.5) | .006 |
| Gestational age at delivery, wks | 38.6 ± 2.3 | 39.05 ± 1.7 | < .001 |
| Preterm birth <37 wks | 64 (12.5) | 75 (7.3) | .001 |
| Preterm premature rupture of membranes | 33 (6.4) | 41 (4.0) | .03 |
| Mode of delivery | .72 | ||
| Cesarean | 148 (28.9) | 287 (28.0) | |
| Vaginal | 364 (71.1) | 737 (71.9) | |
| Placental abruption | 6 (1.2) | 3 (0.3) | .03 |
| Preeclampsia | 22 (4.3) | 27 (2.6) | .08 |
| Small for gestational age | 30 (5.9) | 67 (6.5) | .60 |
| Apgar score <5 at 5 min | 5 (0.9) | 3 (0.3) | .08 |
| Cord pH <7.0 a | 4 (0.8) | 15 (1.5) | .27 |
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