The purpose of this study is to determine if the use of slush nitrogen results in higher post-thaw survival rates for oocytes compared to conventionally used liquid nitrogen.

The study is a randomized, controlled trial which seeks to characterize oocyte survival rate following vitrification and subsequent warming with both slush nitrogen and liquid nitrogen. After warming, fertilization and embryology outcomes will be assessed.

Oocyte donors and a single donor sperm source will be utilized. Following enrollment, oocyte donors will undergo ovarian stimulation and oocyte retrieval. The mature (MII) oocytes obtained for each patient will then be assigned a number and randomized via block randomization to one of two treatment groups (liquid nitogen or slush nitrogen). Through block randomization, this will create a paired population of oocytes so that half of each patient's oocytes will undergo cryopreservation with liquid nitrogen and half will undergo cryopreservation with slush nitrogen. Timing of hyaluronidase stripping and vitrification will be consistent with current institutional clinical practice for oocytes.

Following cryopreservation, oocytes will be warmed and intracytoplasmic sperm injection will be performed and subsequent embryology outcomes tracked.

Major Inclusion Criteria for oocyte donors:

1. Age 30 years or younger who meet institutional criteria to be an oocyte donor.
2. Patients who agree to donate oocytes for research purposes and agree to the study protocol.
3. The subgroup of oocyte donors age 35-38 will not be part of the primary analysis, and therefore will not be required to meet the age criteria listed above. The older group of donors will be chosen as the clinical discretion of the study investigators. No specific ovarian reserve cutoffs will be required, although factors related to ovarian reserve will be taken into consideration.

Major Exclusion Criteria for oocyte donors:

1. Under 18 years old
2. All patients who do not voluntarily give their written consent for participation.
3. Patients who do not meet standard institutional criteria for oocyte donation.
4. A diagnosis of polycystic ovarian syndrome (PCOS).

embryologist assessing oocyte survival (primary outcome) is blinded