During each study day, eligible participants will randomly receive 1 of 36 test glycine intakes during both mid and late pregnancy, ranging from 10 to 100 mg/kg/d. Each subject may participate in 2 study days between 20-30 weeks gestation and 2 study days between 31 - 40 weeks gestation, for a total of 4 study days. The test glycine intakes will contain a crystalline L-amino acid mixture based on the composition of egg protein, except for glycine. The test glycine intake will be administered in an experimental diet, along with enough non-protein energy to meet 1.7 x resting energy expenditure of each participant. The diet will be provided as 8 meals, each representing 1/12th of the participant's daily intake requirement. These meals will be provided over the course of an 8 hour study day. A stable isotope tracer will be added to the 5th - 8th meal. We will measure the rate of oxidation of this tracer in expired breath (F13CO2), and flux of this tracer by its enrichment in urine.