Individual growth patterns in the first trimester: evidence for differencein embryonic and fetal growth rates

Objective To evaluate individual fetal growth during the first trimester in pregnancies resulting from spontaneous and in vitro fertilization (IVF). Methods The growth of 11 fetuses conceived by spontaneous fertilization (kn own dates of ovulation) in nine patients and 15 fetuses conceived by IVF in 12 patients were evaluated at weekly intervals from 6 weeks, menstrual age , to 14 weeks. Fetal length was determined at each examination. Measures of fetal length included the crown-rump length (CRL), maximum straight line l ength (MSLL) and maximum axial length (MAL). Comparisons of CRL and MSLL to MAL were carried out. The MSLL was used as the measure of length except wh en the MAL was available. Linear and quadratic Junctions were fitted to the complete data sets of individual fetuses in the two groups. Individual dat a sets from ten fetuses in each group were then divided into early and late growth phases, and linear functions were fitted to each data subset. Start points and pivotal points for each fetus were estimated from the coefficen ts of these two functions. Growth in these two groups of fetuses was compar ed, on the basis of slope values. Results Evaluation of length measures indicated that, before 8 weeks, only MSLL could be measured. After 8 weeks, all three measures could be obtained , with the MAL being the largest. Both the linear and quadratic models perf ormed well with individual data sets (mean R-2(+/-SD): linens 98.1 (1.0)%; quadratic 99.4 (0.4)%), with no differences found between spontaneous and I VF groups (maximum possible differences in mean slopes (95% probability): 5 -8%). Similar findings were obtained for the early and late growth phase da ta subsets. Slope values in the early and late growth phases showed low var iability (CV: early 13.5%; late 11.6%), but were significantly different (e arly 0.72 (+/-0.10 SD) cm/week; late 1.21 (+/- 0.14 SD) cm/week). The mean start point was 5.9 (+/- 0.3 SD) weeks' menstrual age, while the mean pivot al point was 9.2 (+/- 0.7 SD) weeks, menstrual age. Conclusions First-trimester growth studies in individual fetuses indicate t hat there is a change in length growth rate between 9 and 10 weeks, menstru al age. This is consistent with a shift in development from organogenesis t o growth. These results can be used for more accurate assessment of first-t rimester growth and may aid in the detection of fetal problems that manifes t themselves as growth abnormalities.