Table 2.
Fetal Growth and the Risk of Spontaneous Preterm Birth in the 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 | 1.19 | 0.97, 1.46 | 0.10 | 1.22 | 0.99, 1.49 | 0.06 | 1.19 | 0.98, 1.44 | 0.08 | 1.22 | 1.01, 1.47 | 0.04 |
| AC | 3,660 | 1.18 | 0.96, 1.45 | 0.12 | 1.20 | 0.98, 1.48 | 0.08 | 1.18 | 0.96, 1.44 | 0.12 | 1.21 | 0.98, 1.48 | 0.07 |
| FL | 3,667 | 1.12 | 0.92, 1.36 | 0.28 | 1.18 | 0.96, 1.44 | 0.12 | 1.12 | 0.91, 1.38 | 0.28 | 1.18 | 0.96, 1.46 | 0.13 |
| EFW | 3,658 | 1.20 | 0.99, 1.47 | 0.06 | 1.26 | 1.03, 1.54 | 0.02 | 1.21 | 0.99, 1.47 | 0.06 | 1.27 | 1.04, 1.54 | 0.02 |
| HC:AC ratio | 3,609 | 0.95 | 0.77, 1.18 | 0.66 | 0.95 | 0.77, 1.18 | 0.66 | 0.95 | 0.76, 1.20 | 0.68 | 0.95 | 0.75, 1.20 | 0.68 |
| AC:FL ratio | 3,658 | 1.03 | 0.84, 1.26 | 0.80 | 1.00 | 0.82, 1.23 | 0.98 | 1.03 | 0.84, 1.25 | 0.80 | 1.00 | 0.82, 1.22 | 0.98 |
| Fetal growth velocity from 20 weeks to 28 weeks | |||||||||||||
| ΔHC | 3,445 | 0.85 | 0.67, 1.06 | 0.15 | 0.86 | 0.68, 1.09 | 0.21 | 0.85 | 0.68, 1.08 | 0.18 | 0.87 | 0.68, 1.11 | 0.21 |
| ΔAC | 3,656 | 0.91 | 0.74, 1.10 | 0.33 | 0.90 | 0.74, 1.10 | 0.32 | 0.91 | 0.75, 1.11 | 0.34 | 0.91 | 0.74, 1.11 | 0.34 |
| ΔFL | 3,662 | 0.73 | 0.60, 0.89 | 0.002 | 0.73 | 0.59, 0.89 | 0.002 | 0.74 | 0.60, 0.90 | 0.004 | 0.73 | 0.58, 0.92 | 0.007 |
| ΔEFW | 3,654 | 0.79 | 0.64, 0.99 | 0.04 | 0.79 | 0.63, 0.99 | 0.04 | 0.80 | 0.64, 0.99 | 0.04 | 0.79 | 0.63, 0.99 | 0.05 |
| ΔHC:AC ratio | 3,433 | 0.98 | 0.81, 1.17 | 0.80 | 0.98 | 0.81, 1.18 | 0.83 | 0.98 | 0.80, 1.19 | 0.83 | 0.98 | 0.80, 1.20 | 0.84 |
| ΔAC:FL ratio | 3,651 | 1.16 | 0.97, 1.38 | 0.11 | 1.16 | 0.97, 1.40 | 0.11 | 1.16 | 0.96, 1.40 | 0.13 | 1.16 | 0.96, 1.42 | 0.13 |
| Fetal biometry at 28 weeks | |||||||||||||
| HCd | 3,490 | 1.02 | 0.82, 1.27 | 0.85 | 1.07 | 0.86, 1.33 | 0.57 | 1.03 | 0.81, 1.31 | 0.83 | 1.07 | 0.84, 1.37 | 0.58 |
| AC | 3,670 | 1.06 | 0.86, 1.29 | 0.60 | 1.08 | 0.88, 1.33 | 0.47 | 1.06 | 0.87, 1.30 | 0.58 | 1.09 | 0.88, 1.34 | 0.44 |
| FL | 3,669 | 0.81 | 0.66, 0.99 | 0.04 | 0.85 | 0.68, 1.05 | 0.12 | 0.81 | 0.65, 1.02 | 0.08 | 0.85 | 0.67, 1.09 | 0.20 |
| EFW | 3,670 | 0.99 | 0.81, 1.22 | 0.96 | 1.04 | 0.85, 1.28 | 0.70 | 1.00 | 0.81, 1.23 | 0.96 | 1.05 | 0.84, 1.30 | 0.67 |
| HC:AC ratio | 3,488 | 0.99 | 0.80, 1.23 | 0.96 | 0.99 | 0.80, 1.22 | 0.93 | 1.00 | 0.79, 1.25 | 0.97 | 0.99 | 0.79, 1.24 | 0.93 |
| AC:FL ratio | 3,667 | 1.23 | 1.01, 1.51 | 0.04 | 1.21 | 0.99, 1.49 | 0.06 | 1.23 | 0.99, 1.54 | 0.06 | 1.21 | 0.97, 1.52 | 0.09 |
Abbreviations: AC, abdominal circumference; CI, confidence interval; EFW, estimated fetal weight; FL, femur length; HC, head circumference; HR, hazard ratio; POP, Pregnancy Outcome Prediction; sHR, ratio of the subdistribution hazards.
a Fetal biometry and growth velocity measures are expressed in z scores estimated in the POP Study. Therefore, hazard ratios are given per 1-standard-deviation increase in fetal biometry or growth velocity measure.
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).