The relationship of initial embryo crown--rump length to pregnancy outcome and abortus karyotype based on new growth curves for the 2-31 mm embryo.

The objective of this study was to determine if measurement of initial crown--rump length (CRL) is helpful in predicting low birth weight, newborn length, spontaneous abortions, or abortus karyotype. We measured CRL prospectively in 837 consecutive singleton pregnancies at the time a heart rate was first detectable with transvaginal ultrasonography and compared these measurements to normal values for the 10th through 90th centiles determined from 227 transvaginal ultrasound measurements in in-vitro fertilization and gamete intra-Fallopian transfer pregnancies with known ovulation dates. The relationship of initial CRL to birth weight and length and to abortion and abortus karyotype was analysed after all pregnancies had delivered. Initial CRL measured after the 28th post-ovulation day was predictive of subsequent abortion, but not of low birth weight or length. The abortion rate was 3.3% [95% confidence interval (CI) 1.5%, 5.1%] when initial CRL > or = 50th centile, compared to 19.4% (95% CI 15.4%, 23.4%) when < 50th centile. Initial CRL was < 50th centile in 13 out of 14 trisomic and in eight out of 10 other karyotypically abnormal aborti. These results indicate that initial CRL measured after the 28th post-ovulation day may help to identify pregnancies at increased risk of abortion due to abnormal karyotypes.