Cryopreservation of human embryos has been a routine procedure since the early 1980s. Until quite recently, it involved a “conventional” slow freezing process which unfortunately resulted in intracellular ice crystal formation which damaged embryos, destroying some and reducing the ability of most to propagate viable pregnancies. pregnancy. The introduction over the last 10-15 years of ultra-rapid freezing or “vitrification” has changed all this.  This  technology involves rapidly  freezing  (literally within the blink of an eyelid), thereby avoiding  intracellular  ice from forming and  thereby preventing cell damage. When compared to the “older approach”,  vitrification improved  post-thaw (warming) embryo  survival from about 60%  to  >90%.

Vitrification involves ultra-rapidly freezing of embryos (and eggs) in a tiny amount (less than 0.1 microliters) of a special vitrification solution, before storing them in liquid nitrogen.

Since embryos failing to develop to the blastocyst stage are virtually always chromosomally abnormal, “incompetent” and thus unworthy of storing, I strongly advocate in favor of for the selective vitrification of expanded blastocysts… about 120-144 hours post egg retrieval.

Staggered IVF (St-IVF): It involves splitting the IVF cycle into two phases (separate cycles)  in order to make it possible to test the embryo for its chromosomal integrity (preimplantation Genetic sampling/testing (PGS/PGT-A). St-IVF, which  allows us to  selectively transfer only chromosomally normal embryos for FET in a subsequent cycle. The embryo biopsy involves removing  several cells (blastomeres) taken from the outer layer (trophectoderm) of the advanced embryo (blastocyst), on the 5th or 6th  day post fertilization. These blastomeres are subjected to  numerical chromosomal analysis (karyotyping)  using a technique such as Next Generation Gene Sequencing (NGS),  which allows for  identification of  ALL 23 pairs of the embryo’s chromosomes. An embryo in which all blastomeres have 46 chromosomes is labeled as euploid blastomere/embryo  has all 46 chromosomes present. When there are  more/or less than 46 chromosomes identified, that blastomere/embryo  is referred to as being aneuploid. A euploid . Such an embryo,  upon being transferred to a “receptive” uterus, is likely to propagate a viable pregnancy in about 50-60% of cases. Blastomeres with more (or fewer than) 46 chromosomes are “aneuploid” and if all blastomeres in the embryo are aneuploid, such an embryo will not be able to propagate a normal baby. In some cases the embryo might comprise some euploid and some aneuploid blastocysts. Such embryos are termed “mosaics” . Some such “mosaic”  embryos are able to autocorrect once placed in the uterus and go on to propagate healthy babies.

The process of St-IVF is as follows:

1: The eggs are harvested, the embryos are subjected to PGS/PGT-A and the day5-6 blatocysts they propagate are vitrified.

2: Euploid and “mosaic” blastocysts are retained in cryostorage. The remainder are usually discarded.

3.The patient(s) is/are informed whether/how many potentially “competent” embryos are available for subsequent FET in an upcoming hormone replacement cycle.

4: FET is performed with one (or two) “competent blastocyst”, in a subsequent menstrual cycle. as described below.

Some  treating Res prefer to vitrify embryos prior to their reaching the blastocyst stage. If in such cases, a decision is made to perform PGS/PGT-A  all thawed embryos should be allowed to develop to the blastocyst stage before being biopsied for karyotyping . The biopsied blastocysts are then held in the vitrified state pending access to the the results of PGS/PGT-A testing.

In some some cases, where blastocysts were vitrified WITHOUT having been subjected to PGS/PGT-A a decision is made to thaw such blastocysts in order to  secondarily biopsy them for PGS/PGT-A. While pregnancies are reported in such cases, I discourage such practice because it weakens the blastocysts and is associated with a significant reduction in live birth rates.

Preparing for the Frozen Embryo Transfer (FET): The recipient’s cycle is initiated with an oral contraceptive (OC), which is later overlapped with a  GnRHa (e.g. Lupron, Superfact/Buserelin/Decapeptyl for 5-6 days. Thereupon the OC is withdrawn and daily  GnRHa injections are continued until the onset of menstruation. At this point the GnRHa is administered in a reduced dosage and intramuscular  estradiol valerate (Delestrogen) is administered every 3 days. The objective is to achieve and sustain an optimal  plasma E2 concentration of 500pg/ml-1000pg/ml  and propagate a 9mm endometrial lining  as assessed by ultrasound examination. Intramuscular and/or intravaginal  progesterone is administered daily starting about 6 days prior to the FET and continued along with twice weekly I.M, Delestrogen until until the 10th week of  pregnancy or until pregnancy is discounted.  .

At SFS oral daily dexamethasone commences with the GnRHa start and continues until a negative pregnancy test or until the completion of the 8th-9th week of pregnancy whereupon it is tailed off over 2 weeks and discontinued. Oral folic acid ( is taken daily commencing with the first Estradiol valerate injection, and is continued throughout gestation. The recipient also receives prophylactic oral antibiotics starting with the initiation of Progesterone therapy, until the day after ET.

Usually we would warm/thaw vitrified blastocysts with the objective of having 1 or 2 for transfer.

Commencing on the day following the ET, the patient inserts a vaginal progesterone suppository daily and this is continued until the completion of the 10th week of pregnancy or until pregnancy is discounted. Selective  immunotherapy (Lovenox and/or Intralipid) is otherwise administered when indicated as with conventional SFS-IVF patients.

As an alternative regimen for women who cannot tolerate intramuscular Progesterone PIO I prescribe one (1) vaginal application of Crinone 8% administered on the 1st day (referred to as luteal phase day 0 – LPO). From the next day, (LP Day 1) Crinone 8% is used twice daily (A. M. and P. M.) until the day of embryo transfer. Crinone 8% is withheld on the morning of the embryo transfer and is thereupon resumed straight after the FET.  

For blastocyst FET’s, the blood pregnancy tests are performed 13 days and 15 days after the first progesterone administration is commenced . Contingent upon positive blood pregnancy tests, and subsequently upon the ultrasound confirmation of a viable pregnancy, administration of I.M progesterone and/or Crinone 8% and twice weekly Estradiol Valerate are resumed and continued until the completion of the 10th week of pregnancy or until a pregnancy is ruled out.

Introduction of ultrarapid freezing (Vitrification) represented s a major “paradigm shift” in the field of human ART. It allowed for cryopreservation of embryos can be performed with relative impunity. Likewise, it represents a “shot in the arm” for  human egg freezing, offering promise of better egg survival rates and subsequent post-warming  pregnancy potential, leading to the proliferation commercial  egg banks for egg-donor IVF as well as the popularization  Fertility preservation (FP).