ембриолог се подготвя да извърши процедура по инвитро оплождане под микроскоп


Assisted reproduction technology - fertilization

Which method is applicable in your case is a decision of your fertility consultant, after a careful workup and assessment of your medical history, previous tests, and current status, as well as additional tests and consultations where necessary. At ‘Nadezhda’, we focus on the personalized approach to every case, together with interdisciplinary work by our team for optimal care and best results.


For IVF, the eggs retrieved by follicle aspiration are delicately cleaned of excess cells and are placed in a special medium in a lab dish (petri dish) labelled with your names. Spermatozoa that have been previously washed, processed and pre-activated in the Andrology lab are added to the eggs at a specific concentration. After this, the petri dish with the fertilisation medium is placed in the incubator. The aim is that one of the spermatozoa eventually fertilises the oocyte by fusing with it after penetrating the layer of cumulus cells surrounding it and its membrane. The next day, the embryologist will check the eggs for signs that fertilisation has occurred – there should be two pronuclei visible, a male and a female one. The fertilised egg is called the pre-embryo or the zygote. Zygotes are then closely monitored and assessed for regular cellular division in the next two to five days. You will be informed in detail about their development in phone calls by our embryologists.


NB! Various functional defects in the eggs or sperm can interfere with fertilisation. You can ask your fertility consultant about the different fertilisation techniques and the advanced procedures that can be applied to oocytes and sperm as part of the IVF cycle.  

An advanced type of in vitro fertilisation is the ICSI method (short for intracytoplasmic sperm injection). It is similar to classic IVF, but the key difference is that a single sperm is injected into the egg by a special technique. First, before fertilisation, the eggs are prepared by a delicate chemical and mechanical treatment removing the surrounding cumulus cells. Also, their maturity and suitability for fertilisation are evaluated under the microscope before the procedure. The technique is performed using micropipettes under microscopic control through a micro-manipulator. 


ICSI is applied most often when dealing with the so-called male factor infertility. More specifically, when the semen analysis of your partner shows low sperm counts, poor motility, poor morphology, as well as in cases when we have retrieved the sperm by a testicular biopsy.

There is no simple answer to this question, and this is why you have to discuss each option with your fertility consultant.


The decision to use one or the other method depends on multiple factors, such as the female partner’s age, whether she has had previous pregnancies, the current cause for infertility, the sperm count, the number of eggs retrieved, their quality, etc.


When there are good semen analysis results and a high number of eggs retrieved, the fertility consultant may choose to apply both methods for in vitro fertilisation combined.

IMSI is actually a modified ICSI procedure. A single sperm is selected under ultra-high microscopic magnification based on morphological criteria – normal form and size and no visible defects in its head, neck, or tail. The sperm is then injected into the egg’s cytoplasm.

See also

Specialised procedures - eggs
Specialised procedures - embryo
Specialised procedures - sperm
Fertility preservation