RESUMO
BACKGROUND: The accurate estimation of gestational age by ultrasound is crucial in prenatal care for the monitoring of fetal growth and development. As changes in maternal childbearing age, body habitus, and ultrasound technology occur, previously published formulas may not be accurate for today's population. OBJECTIVE: This study aimed to develop new formulas for calculating the gestational age based on a first-trimester ultrasound scan and to compare the new formulas with preexisting formulas. STUDY DESIGN: This study was a single-center, retrospective observational study that included pregnancies conceived using in vitro fertilization. The pregnancies had known dates of embryo transfer and multiple standard ultrasound examinations in the first trimester of pregnancy. Pregnancies ending with a miscarriage or termination in the first trimester of pregnancy were excluded. A polynomial regression analysis was performed to determine the optimal model that represented the relationship between gestational age and crown-rump length. The models were evaluated using systematic error, random error, absolute difference of the calculated gestational age and actual gestational age, and proportion of estimation within 0 and 2 days of the known gestational age. The optimal model was chosen and compared with preexisting formulas. RESULTS: Overall, 1436 ultrasound results were included in the analysis. The analysis produced 3 models: linear, cubic, and quadratic models with correlation coefficients of 0.968, 0.989, and 0.991, respectively. The cubic formula was superior to the linear and quadratic formulas concerning systematic error, random error, absolute difference, and proportion of estimation within 0 and 2 days. The new formula had a lower systematic error, random error, and mean absolute difference (0.06%, 2.43%, and 0.97 days, respectively) and the highest proportion of estimation within 0 and 2 days (37.4% and 93.5%, respectively) than previously published formulas. CONCLUSION: The formula proposed in this study followed a cubic model and seemed to be able to more accurately estimate gestational age in the first trimester of pregnancy based on crown-rump length compared with previously published formulas.