Freezing – sperm, eggs and embryos

Freezing technology enables maximisation of treatment as well providing options for medical and social reasons. At Repromed we offer the following treatments.

Sperm freezing

Reasons for sperm storage can range due to a number of factors. Sperm can be frozen prior to treatment and used as ‘back-up’ if decreased sperm parameters are noted or anxiety surrounding performance on the day of treatment is predicted.

Sperm freeze is also useful for couples where the male partner is likely to be away from home at time of female ovulation, having frozen sperm in storage enables the treatment to go ahead at any time.

Egg freezing

Why freeze eggs?

There are two primary reasons why people may wish to consider freezing their eggs.

Firstly, those who are planning to undergo medical treatment that may harm or reduce their egg reserves (e.g. chemotherapy), are offered egg freezing as a means of Medical Fertility Preservation.

Secondly, some are choosing to freeze their eggs in an effort to safeguard themselves for the future, against the age-related decline in fertility that naturally occurs. These cases we refer to as Social or Elective Fertility Preservation.

People are born with all the eggs (oocytes) that they need until menopause. Unlike sperm, which is being constantly produced, eggs start to decline in number from the time of birth. Only about 400 eggs will normally be ovulated from the ovaries in a person’s lifetime.

Maximum fertility occurs in the twenties and by the time a person reaches 35 years old, their fertility has declined by half. This drop in fertility is due to the ageing process, when chromosome abnormalities in the eggs also become increasingly prevalent.

By the time a person is 37 years old the proportion of embryos that are produced with normal chromosomes is thought to be only 50%.

The client will undergo controlled ovarian hyper-stimulation to increase the number of follicles growing on the ovaries. This will maximise the number of eggs available for retrieval. Every person is different and responds differently to treatment, so the number and quality of eggs harvested will vary.

Once the eggs have been collected, the support cells around the eggs are removed. At this time, the maturity of the eggs are determined. Only mature eggs are suitable for freezing, therefore not all the eggs retrieved may be frozen. Mature eggs are frozen by first exposing them to a cryo-protective solution and then freezing very rapidly using a process called vitrification.

Each egg is individually frozen and stored. Similar to freezing embryos, once the eggs are frozen they are stable for an indefinite period.

Thawing involves the quick warming of the egg to 37 degrees Celsius and the removal of the cryo-protective solution. After a short period of recovery, the eggs are then suitable for insemination by injecting a single sperm into the egg.

How successful is egg freezing?

Recent international research has indicated that, on a per thawed oocyte basis, the use of frozen eggs has the potential to give similar results as using freshly collected eggs. Published results suggest a single pregnancy may be produced on average from about 10-15 oocytes.

The number and quality of eggs collected and the overall success rate for the procedure depends on the person’s age, the reason for the treatment and other factors. There are instances where no eggs may be collected or suitable for freezing despite everyone’s best efforts.

Predicting your chance of having a baby based on age and number of eggs collected

Each curve in this figure shows the percentage likelihood that a person of a given age will have at least one baby following a future thawing of their eggs.

Based on this table, for a 75% chance of having a baby, you will need an estimated minimum number of eggs at the following ages:


Like any invasive procedure, egg freezing does come with associated risks. Vitrification, a process of snap freezing has been used for embryos for relatively longer period of time than for eggs. Egg freezing is a relatively new procedure and there is currently not a large number of studies to address long term implication of freezing eggs.

Eggs themselves are naturally at a very fragile phase when they are vitrified in comparison with their embryos counterparts, and hence the survival rate of eggs is lower than what is expected for embryos. The survival of the eggs depends on their quality and not the quantity.

Pregnancy rates and miscarriage rates are influenced by the age of the egg, but there are many other pregnancy complications such as pre-eclampsia and age of the person at the time of pregnancy that can also impact the outcome, and freezing eggs does not mitigate these risks.

View our ReproFacts sheet which contains more information on elective egg freezing. 

Embryo freeze and FET

At Repromed our scientists will freeze any good quality embryos following your fresh embryo transfer. These embryos will remain in storage until you would like them thawed and transferred, a procedure known as a Frozen Embryo Transfer (FET).

Due to higher survival rates we freeze all embryos at the blastocyst stage (day 5 and/or 6) using a fast freeze technique termed vitrification.

What is vitrification?

Vitrification is an ultra rapid freeze method that bypasses ice crystal formation. Vitrification is reported to be a superior method of freezing than conventional slow freezing where survival rates range from 70-80% giving rise to increased rates in pregnancy outcome.

Frozen Embryo Transfer (FET)

FET stands for Frozen Embryo Transfer. Following egg collection in an IVF cycle, the team in the lab will freeze any surplus high quality embryos. These embryos remain frozen in storage until the woman or couple are ready to have the embryos thawed and replaced.

Preparation for this involves either ovulation monitoring of a natural cycle, or a programmed cycle with the addition of oestrogen and progesterone hormones.

The embryo is thawed when the uterus is predicted to be at its most receptive phase for implantation. The embryo is then placed in a very fine catheter and inserted through the cervix, in a procedure which is similar to having a smear test.

Like most aspects of fertility, the likelihood of a successful pregnancy from an FET cycle is dependent on the age of the woman when her embryos were created. The graph shows the most current results from Australasian clinics and you can see that the FET cycles in blue declines at a similar rate to the Fresh embryo transfer cycles.

Frozen transfers have a higher rate of pregnancy for women over 35 years old. This can be due to a) the embryos are frozen when the women were slightly younger, and b) they are transferred in an unstimulated cycle where the hormone levels are more natural.

All semi-surgical procedures are carried out in affiliation with Repromed’s DAA accredited surgical unit ACSS (Auckland City Surgical Services) within the same building and using our Repromed doctors.

For some women, a ‘freeze-only’ IVF cycle may be recommended if their doctor is concerned about the effect of high hormone levels on their health or uterine lining. The beauty of waiting to transfer these embryos in a thaw cycle is it gives the body time to return to a natural state when the uterus is more receptive to an implanting embryo.  Only high quality embryos are frozen and their survival rate is almost 100% which can often translate into higher pregnancy rates than fresh IVF transfers. Learn more about the benefits of a freeze-only cycle here.

And finally, FET cycles do have the benefit of requiring no injections and less blood tests. 

Transferring embryos between clinics

Transferring frozen embryos between clinics is a very simple process. If you have embryos stored at a different clinic but would like treatment at Repromed please get in touch with us.


If you would like to discuss your personal situation please get in touch with us. For new clients, we offer a free 15 minute phone consultation with a fertility doctor. Take the first step today.


Megan Black

Nurse Manager


Megan leads the nursing team through the continually changing face of IVF. She works in a multidisciplinary team, providing the essential organization between the doctors and laboratory and ensuring communication between all departments.

Megan started her IVF nursing career in the United Kingdom, working in two large London clinic’s before returning to New Zealand. She is also the Secretary of Fertility Nurses of Australasia.

I love working with people and see nursing as a vocation, not a job. I usually spend my downtime absorbed in a good book and planning my next travel adventure.