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Ovarian Stimulation

The number of oocytes recovered from each woman in different cycles is variable. There could be as little as one or as many as 88 ( we have collected this number in our unit) this may result in large number of embryos. These extra embryos are kept frozen for future use.

Before the cryopreservation procedure was widely available, all these embryos were allowed to grow in the laboratory by repeated division for up to five to six days. These embryos ultimately died. This was a terrible waste. Despite such advances in science it is still not possible to identify the embryos that are most likely to result in a successful pregnancy. Replacement of more than two or more embryos greatly increases the chances of multiple pregnancies with its associated risk. Multiple pregnancies is associated with increased risk of early miscarriage, high blood pressure, bleeding in mid and late pregnancy due to the placenta being in the lower part of the uterus, pre-term labour and foetal abnormality etc. So the risk to the mother and the baby are both increased in multiple pregnancies. Another important use of cryopreservation is in cases of ovarian hyper stimulation syndrome (OHHS) when none of the resulting embryos are replaced but are cryopreserved and used in a subsequent cycle.

During cryopreservation, the embryos are placed in a special plastic tube in pairs. The tube is sealed at both the ends. These are identified by the woman's name, patient number and colour coded. All the tubes containing the woman's embryos are gradually cooled in liquid nitrogen to minus 196oC in a special machine. These tubes are then placed in metal "canes" and then lowered into large flask containing liquid nitrogen. The level of liquid nitrogen is periodically checked and topped up. The embryos are thus kept in storage in liquid nitrogen until required.

The embryos could be stored indefinitely. But from the practical point we intend to store the embryos for a maximum of five years unless the couple make a special request for extension of the storage. A small charge is made for the storage of embryos per year. When a woman returns to have the frozen embryos transferred, there is no need for the ovarian stimulation or the operation to recover the eggs, as we have stored the frozen embryos. However it is necessary to control the pituitary hormones by a single injection. Two weeks later a scan and a blood test are done to confirm the hormonal control. Oestrogen tablets are now given for ten to fifteen days to develop the endometrium before the embryos are returned. The endometrial thickness and quality are assessed by ultrasound scan. Now hormonal suppositories are given (progesterone) along with the oestrogen tablets. The embryos are thawed either on the day before or on the day of embryo transfer depending on what stage the embryos were frozen. Only about 85% of the frozen embryos are successfully thawed. Some embryos are lost during freezing and thawing process. It is not possible to see the quality or the viability of the embryos until they are thawed. In some cases we might replace the embryos in a natural cycle after confirmation of ovulation.

The result of animal studies and the result of frozen thawed embryo replacement in humans suggest that freezing and successful thawing in no way damages the embryos or babies born from this method. The pregnancy rates by frozen embryos in our centre are comparable to pregnancy rates by fresh embryo transfer.