Jason Cordell Birnholz MD, Presenter: Nothing to Disclose

Ultrasonic measurements are used routinely for gestational age (GA) determination and forward projection of an expected average (40 week) due date. Ultrasound has been established as the most accurate form of dating in spontaneous pregnancies.These  data can be used to back project the date on conception with comparable precision. 

The patient population consists of 1186 patients delivering healthy infants who were well staged prior to 20 weeks ga and  for whom data was available for maternal prepregnancy height and weight, parity, standard ultrasonic measurements (BD, OFD, AD, FEM), exam date, fetal/infant gender, delivery date, and birthweight.The vast majority of these patiients reside within a 100 mile radius of metropolitan Chicago. Data was entered prospectively, retrieved from exam reports, and analyzed retrospectively. All ultrasound exams were performed  at our facility; ultrasound measurements were made by a single examiner, following a fixed protocol, and using the same high performance, low noise imaging system for patient.

There were 545 female fetuses and 641 males. There was a significant preponderance of females to males conceived in April and August, and for males vs females in September.There were fewer conceptions for both in March than any other month. The correlation coefficient between length of gestation and BMI was .088 for females  and .006 for males and .126 for first pregnancies producing girls. The average length of pregnancy for females by parity was: 273.42 +/- 12.23, 272.35+/-9.46, 271.37+/-12.31 and for males:275/08+/-11.61, 273.24+/-12.10, 273.44+/- 9.47

Length of Gestation and birthweight are covariables for fetal growth. Female fetuses tend to have slightly but significantly shorter gestational durations than males and more influenced by maternal BMI consistent with an hormonal influence.  There were  disproportionate incididences of male and female conceptions by month. There was a striking preponderance of male fetuses in our study group, believed attributable to community referral practices and currently under investigation.

Conventional fetal ultrasound measurements can disclose unsuspected demographic  patterns of fetal growth.

Birnholz, J, Observations Arising from Ultrasound Determination of the Date of Conception in a Referral Population of Pregnant Women.  Radiological Society of North America 2011 Scientific Assembly and Annual Meeting, November 26 - December 2, 2011 ,Chicago IL. http://archive.rsna.org/2011/11014066.html