This is a longitudinal prospective study wherein all subjects will be examined at three time points within a single menstrual cycle: the early follicular phase (\~day 2), periovulation (\~day 12), and the luteal phase (\~day 21)-based on an approximate 28-day cycle length. The actual days of visits will be adjusted for the length of the woman's menstrual cycle. At least 35 non-pregnant women ages 18 to 44 years will be recruited as a convenience sample of eligible women in the State College area and surrounding towns within Centre County, Pennsylvania, USA. If a woman responds to the recruitment, she will be asked a set of pre-screening questions to determine eligibility. If a woman is thought to be eligible, she will complete a verbal consent for the screening (and the actual screening will occur at the first visit). Then she will be invited to the Clinical Research Center (CRC), a service unit in Penn State's Clinical and Translational Science Institute, for full screening and participation. Before scheduling the visit, the woman will be asked for the expected date of the first day of her next menstrual period. The woman will then be tentatively scheduled for her first visit at the CRC approximately 1 day after this date (avoiding weekend days).

As part of the preparation for study visits, the woman will be asked to fast the night before her visit. Women will also be asked to avoid alcohol intake for 12 hours preceding their scheduled visits to the CRC, and to drink plenty of water. On the morning of the visit, the woman will arrive at the CRC, go through complete screening (including height, weight, blood pressure, and pregnancy test). If eligible for the main study, the woman will then go through the consent process for the main study. If the woman gives consent to participate, the investigators will then proceed with study measurements (including a questionnaire and body composition measurement).

Following these measurements, the study nurse or doctor will insert an IV into the vein of one arm and an initial baseline blood draw will be taken (\~15-20 ml, in several tubes). This blood will be used for biomarker assessment (hemoglobin, hematocrit, vitamins, minerals, etc) and as a blank sample before the dye injection. Next, indocyanine green (ICG) will be used to measure plasma volume. A solution will be prepared using 25 mg of ICG lyophilized powder dissolved in 10 ml of sterile water. The investigators will draw enough solution to inject 0.25 mg/kg body weight into a syringe. The syringe will be weighed to measure the exact amount. Then the solution will be injected via the IV and flushed with saline. After 2 minutes, five \~3 ml blood samples will be drawn at intervals of 45 seconds over the next 3 minutes. Expected total volume of blood drawn at the visit will be \<35 mL.

At the end of this first visit the participant will be given a home-based fertility monitor to take home to track her cycle. She will be instructed on how to use the monitor at home but the research team will maintain contact with her on a regular basis to ensure that she is using the monitor correctly. She will conduct a daily urine test with the monitor, which uses urine hormone concentrations to track the stage of the menstrual cycle for the participants by displaying low, high, or peak fertility. The second and third visits will be scheduled using an algorithm that takes into consideration the woman's menstrual cycle length and results from a home-based urine fertility monitor.

At the third visit, the participant will return the monitor to the CRC and the urine test results, which are stored in the monitor, will be retrieved. At the end of the third visit her study participation is complete but the investigators will keep in contact with her to record the first date of her next cycle (to calculate actual cycle length).