Phenylalanine and tyrosine are aromatic amino acids that people need to obtain in their diets. Amino acids are the building blocks for protein, are are therefore required to form tissues in the body. It is well known that pregnant women require more protein in their diets, but the exact amount for each amino acid is undetermined. To find out how much of phenylalanine and tyrosine pregnant women require, the investigators plan to study pregnant women in early and late gestation with a modern minimally invasive technique.

Following a study to determine phenylalanine requirements in pregnant women, the investigator's purpose for this study is to determine the phenylalanine and tyrosine requirements (also know as the total aromatic amino acid requirement) in healthy pregnant women as part of a Doctoral Thesis project. The determined requirements can be taken into account when giving dietary recommendations, to ensure healthy pregnancies. The hypothesis is that the current recommendations for phenylalanine and tyrosine intake given to pregnant women are underestimated. The investigators also hypothesize that the phenylalanine and tyrosine requirements at early stages of pregnancy will be lower than the requirements towards the later stages of pregnancy.

Current dietary intake recommendations for the aromatic amino acids are based on factorial (mathematical) calculations and do not account for requirement differences between early and late pregnancy. Static recommendations throughout all stages of pregnancy are not appropriate because of the multitude of adaptations the human body goes through during gestation. Whether phenylalanine and tyrosine requirements differ between different stages of gestation is unknown. The DRI gives a recommendation for total aromatic amino acids of 36 mg/kg/d during pregnancy. The results from this study would allow for a more accurate recommendations in future DRIs.

The investigators will recruit healthy pregnant women aged 20 to 40 in their 13th-19th week of gestation or 33rd to 39th week of gestation. Once they get in contact with the lab, an initial visit (pre-study) will take place. Here, the participants will be measured for their eligibility and taken through the consent form. They have to participate in a pre-study for each stage of pregnancy, if they wish to participate in both stages. If they meet all the criteria and agree to participate, they will return for up to four study days per stage of 8 hours each. Therefore, if they are recruited during late pregnancy they are only able to participate in four study days in total. During these study days they will be fed protein shakes containing the stable isotope, and then oxidation of this isotope will be measured to determine a breakpoint (requirement) of phenylalanine. Breath samples, urine samples, and a small blood sample will be taken during the study day.

Inclusion Criteria:

• over the age of twenty
• under the age of forty
• in overall good health
• pregnant with a single child

Exclusion Criteria:

• Be under the age of twenty
• Be over the age of forty
• Be pregnant with more than one child
• Have a history of cardiovascular disease, endocrine disorders, or metabolic disorders
• Have recently lost a substantial amount of weight
• Have been pregnant with another child in the last 18 months
• Be substance dependent
• Be allergic to eggs
• Have severe nausea through the pregnancy
• Have gestational diabetes
• Have a history of spontaneous abortions
• Have had an unhealthy pre-pregnancy body mass index