Introduction Accurate pregnancy dating is important for many aspects of obstetric care at individual level as well as population level. Traditionally, pregnancy dating has done by adding 9 months and 7 days to the last menstrual period (LMP) using Naegele’s formula. Determination of gestational age by ultrasound is more precise. Most commonly used parameters are mean sac diameter, gestation sac volume, crown-rump length (CRL), biparietal diameter (BPD), head circumference (HC) and femur length (FL). After 24 weeks, gestational age cannot be accurately determined by ultrasound scans. The biological variability of CRL is small and growth is very rapid. There are many factors that can affect CRL such as measurement errors, differences in growth rates between individuals, fetal sex and maternal conditions such as diabetes mellitus. A correctly performed measurement of CRL is the most accurate way of estimating the gestational age in early pregnancy from 8 weeks to 13 weeks + 6 days. Objectives Our study aims were to prepare a new Crown Rump Length chart with Sri Lankan population data and to compare new CRL chart with existing intergrowth CRL chart. Method Prospective observational study with recruitment of subjects by Quota sampling technique was carried out from April 2015 to March 2016. Spontaneously conceived uncomplicated singleton pregnancies with normal Body Mass Index (BMI) 18.5 - 23 kg/m2 were recruited at the time of registration to antenatal care. Consenting woman with known LMP with regular cycles in preceding 3 months were undergone ultrasound examination only once at gestational age (GA) ranging from 8 weeks to 13 weeks + 6 days. If ultrasound dating was different from LMP dating by more than 5 days in pregnancies with POA less than 9 weeks and dating differences more than 7 days in pregnancies between 9 weeks and 14 weeks were excluded. Pregnancies complicated with uncertain viability, congenital anomalies and spontaneous miscarriage were excluded from statistical analysis. Data collection done with pre-tested interviewer administered form and analysis was carried out using the Statistical Package for Social Sciences (SPSS) version 21. Results A total of 653 subjects were recruited for the study and 557 turned up for ultrasound dating assessment. Dating discrepancy and multiple pregnancies excluded 31 subjects, uncertain viability and spontaneous miscarriage excluded 15 subjects resulting 511 subjects for final statistical analysis. The mean CRL increased with GA almost linearly from day 56 to 97. Conclusions We have produced new CRL chart based on Sri Lankan data and it can be used for clinical practice in Sri Lanka. There is no statistically significant difference between our CRL chart and intergrowth CRL chart.
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