Table 3.
Risk of Spontaneous Preterm Birth According to Measures of Fetal Growth, Estimated Using an International Reference Standard, POP Study, Cambridge, United Kingdom, 2008–2012
| Measurea | No. of Participants | Cause-Specific Regression Analysis |
Competing-Risks Regression Analysis |
||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Unadjusted |
Adjustedb |
Unadjusted |
Adjustedb |
||||||||||
| HR | 95% CI | P Valuec | HR | 95% CI | P Value | sHR | 95% CI | P Value | sHR | 95% CI | P Value | ||
| Fetal biometry at 20 weeks’ gestation | |||||||||||||
| HC | 3,619 | 1.24 | 0.95, 1.62 | 0.12 | 1.28 | 0.98, 1.67 | 0.07 | 1.24 | 0.97, 1.59 | 0.09 | 1.28 | 1.00, 1.64 | 0.05 |
| AC | 3,660 | 1.22 | 0.94, 1.57 | 0.13 | 1.25 | 0.97, 1.62 | 0.09 | 1.22 | 0.95, 1.57 | 0.12 | 1.26 | 0.98, 1.61 | 0.08 |
| FL | 3,667 | 1.14 | 0.89, 1.45 | 0.29 | 1.21 | 0.95, 1.55 | 0.12 | 1.14 | 0.89, 1.46 | 0.30 | 1.22 | 0.94, 1.57 | 0.13 |
| Fetal growth velocity from 20 weeks to 28 weeks | |||||||||||||
| ΔHC | 3,445 | 0.87 | 0.67, 1.11 | 0.26 | 0.89 | 0.69, 1.15 | 0.38 | 0.87 | 0.67, 1.15 | 0.33 | 0.90 | 0.68, 1.19 | 0.46 |
| ΔAC | 3,656 | 0.92 | 0.75, 1.14 | 0.47 | 0.93 | 0.75, 1.15 | 0.49 | 0.93 | 0.75, 1.15 | 0.49 | 0.93 | 0.74, 1.16 | 0.52 |
| ΔFL | 3,662 | 0.72 | 0.58, 0.89 | 0.002 | 0.72 | 0.58, 0.90 | 0.003 | 0.72 | 0.58, 0.91 | 0.006 | 0.73 | 0.57, 0.93 | 0.01 |
| Fetal biometry at 28 weeks’ gestation | |||||||||||||
| HCd | 3,490 | 1.02 | 0.82, 1.26 | 0.86 | 1.06 | 0.86, 1.32 | 0.57 | 1.03 | 0.81, 1.30 | 0.83 | 1.07 | 0.84, 1.36 | 0.59 |
| AC | 3,670 | 1.05 | 0.86, 1.28 | 0.62 | 1.08 | 0.88, 1.32 | 0.48 | 1.06 | 0.87, 1.29 | 0.59 | 1.08 | 0.88, 1.33 | 0.46 |
| FL | 3,669 | 0.82 | 0.67, 1.00 | 0.05 | 0.86 | 0.70, 1.05 | 0.13 | 0.83 | 0.67, 1.02 | 0.08 | 0.86 | 0.69, 1.08 | 0.20 |
Abbreviations: AC, abdominal circumference; CI, confidence interval; FL, femur length; HC, head circumference; HR, hazard ratio; IQR, interquartile range; POP, Pregnancy Outcome Prediction; sHR, ratio of the subdistribution hazards.
a Fetal biometry and growth velocity measures are expressed in z scores with respect to the INTERGROWTH-21st Project reference (41). HRs are given per 1-standard-deviation increase in fetal biometry or growth velocity measure according to the international reference. Median percentiles in the POP Study were 52.8 (IQR, 32.8–71.7) for HC, 66.6 (IQR, 44.4–83.0) for AC, and 48.0 (IQR, 30.7–73.3) for FL at 20 weeks’ gestation and 73.4 (IQR, 46.9–90.2) for HC, 66.1 (IQR, 40.3–86.7) for AC, and 53.2 (IQR, 27.4–79.1) for FL at 28 weeks’ gestation. Median percentiles of change from 20 weeks to 28 weeks were 70.8 (IQR, 50.0–86.1) for ΔHC, 51.9 (IQR, 27.1–74.3) for ΔAC, and 53.8 (IQR, 27.6–76.4) for ΔFL.
b Adjusted for maternal height, age, body mass index, marital status, previous spontaneous abortion, ethnicity, smoking status, age at leaving full-time education, and Index of Multiple Deprivation score.
c All P values in the table were derived from the z test specific to each method (i.e., cause-specific or competing-risks regression). All tests for significance were 2-sided.
d Departure from proportionality for week 28 HC was detected in both the unadjusted cause-specific regression analysis (Schoenfeld test: P = 0.01) and the competing-risks regression analysis (z test for interaction with follow-up time: P = 0.008).