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Given a passage with a context and an answer, construct a question on the basis of the information present in the passage. Construct the question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique (iv) it answer uses text phrases from the passage. Avoid creating questions that (i) can't be answered correctly without actually understanding the passage and (ii) uses the same words or phrases given in the passage.

Context: To determine fetal thymus growth and its relationship with fetal weight and cord blood T-regulatory cells in a prospective study. Assessment of fetal immune organs by ultrasound could provide a screening approach to identify fetuses at risk of impaired postnatal immunity.', 'Thymus size was measured with four ultrasound techniques. The approaches with lowest coefficient of variation (thymus transverse diameter, 3 vessel edge) were used to longitudinally assess fetal and thymus growth in 137 cases at four time points between a gestational age (GA) of 13 and 37 weeks. Cord blood at birth was analyzed by flow-cytometry to evaluate the frequency of regulatory T (Treg) cells.
Answer: Fetal thymus growth is significantly correlated with fetal weight (GA 23-25 weeks r=0.40, p<0.01; GA 28-30 weeks r=0.21, p=0.04, GA 35-37 weeks r=0.56, p<0.01). We observed an inverse correlation between fetal thymus size at GA 23-25 weeks and cord blood Treg cells (r=0.37, p=0.01). Thymus growth occurs in a linear fashion throughout pregnancy and can be reliably measured using ultrasound. Our findings of an inverse correlation between thymus growth and Treg cells in cord blood suggests a link between fetal growth, thymus development and immune-status at birth.
Output:
Does fetal thymus size in human pregnancies reveal inverse association with regulatory T cell frequencies in cord blood?