IVF patients often don’t understand the difference between
follicles ( which are affectionately called
“follies” ) and eggs . Doctors are sometimes
responsible for this confusion , because we usually loosely refer to the
follicles we see on your ultrasound scan as eggs. This is especially true
during IUI cycles; or when the scans are being done by a sonographer or
technician. When she sees that your ovaries have responded well to the
superovulation, she will often say – Good, your eggs are growing well.
In reality, eggs are microscopic structures which cannot be
seen on ultrasound scans. They are only 100 microns ( 0.1 mm) in size, and can only be seen by the
embryologist in the IVF lab when he scans the follicular fluid the doctor sends
him during the egg retrieval under the stereozoom microscope .
Ultrasound scans allow us to track the growth of the
follicles in your ovaries. Follicles are
seen as tiny black bubbles on the scan, because they contain follicular fluid.
They range in size from 4 mm – 25 mm; and the eggs grow within these follicles.
However, not all follicles contain eggs ,
which is why the correlation between the number of follicles seen on the IVF ultrasound
scans and the number of eggs which are actually retrieved is not perfect.
When all the follicles grow at the same rate ( a synchronous
cohort), then timing the HCG is much easier. However, often the growth is asynchronous;
and some follicles may become big while the
others remain small. It can be very difficult for the doctor to time the HCG in
Thus, you may be disappointed when the doctor gets fewer eggs
as compared to the number of follicles counted at the time of giving the HCG trigger.
This is because some follicles are small and don't contain eggs ; while others
are large , and have formed cysts which do not have an egg. It’s not possible
to differentiate between a cyst and a large follicle based on the ultrasound
scan images. It’s only after the doctor sends the follicle fluid to the
embryologist in the IVF lab will we know whether there is an egg in it or not.
Another possibility for not getting as many eggs as expected
is that the doctor may have technical problems during egg retrieval , and may
not be able to reach the follicle ( for example, when the patient is obese and
the procedure is being done without general anesthesia) . Some doctors may not
do a good job with aspirating each follicle, and may fail to collect the egg, because
they do not flush the follicle completely and the egg remains stuck to the
follicular wall ; or because there is too much blood; or because the
embryologist may not be experienced at identifying the egg. This is especially
true when there are only a few eggs; or if the doctor is not experienced; or if
there are too many cases scheduled on that day and the doctor is in a hurry to
complete your procedure.
Sometimes, we may also get more eggs than the number of follicles
counted on the scan. This is because doctors may not bother to diligently count
all the follicles seen on the ultrasound scan ; or that some follicles are
hiding behind the others , which is why they may not be seen during the scan.
Even if the doctor does manage to retrieve all the eggs, do
remember that not all the eggs will be mature ; and that not will fertilize .
Because we are interested in the quality and the quantity of
the eggs, we also track the blood levels of estradiol during the IVF cycle.
Estradiol is not produced by the egg, but by the granulosa cells which line the
follicle. As the follicle matures, it increases in size and the number of granulosa
cells also increase, as a result of which the estradiol levels rise progressively.
Estradiol levels help us to determine
when the follicles are mature, so we can time the HCG trigger properly, and maximize
the number of mature eggs we retrieve. While the correlation between estradiol
levels and the number of mature eggs collected is quite good, it’s not always perfect.
This is why IVF doctors need a lot of experience and expertise when
interpreting these scans . This is especially true in older women; patients
with PCOD; and those with poor ovarian reserve, as the margin for error in
these patients is very thin.