Its 2021. Top IVF centers across the globe are performing embryo transfers with frozen embryos
only. During a frozen embryo transfer, doctors monitor the thickness of the uterine lining before
ovulation or before starting progesterone. The concern is that women with a thin uterine lining
may have a lower chance for pregnancy. But what is too thin? Some programs think that
the lining must be 8 or 9 mm thick. Other programs use a thicker or a thinner cutoff.
There is obviously a lot of confusion about this. Today, on InfertilityTV I will show you evidence
that excellent pregnancy rates are achievable with thinner linings than you may think. Stay tuned
First off I want to caution you not to mix different concepts together.
Embryos can be transferred in the same cycle as an egg retrieval. This is known as a fresh
transfer. We are not talking about fresh transfers today. More commonly, the embryos are frozen and
transferred at a later time. Embryos can be frozen and subsequently transferred at different stages
in their development. In 2021, most programs freeze embryos after 5 to 6 days of development at
the blastocyst stage. Frozen embryo transfers at the blastocyst stage is what we will focus on. For
these frozen embryo transfers, it is important to place the embryos into a uterus that is prepared.
There are two phases of uterine preparation. During the first phase,
called the proliferative phase, the uterus is being exposed to estrogen from the ovaries or
from medication. During this phase the uterine lining grows thicker. During the second phase,
called the secretory phase, the uterus is being exposed to progesterone. During this phase,
the lining does not grow thicker and often gets thinner as the lining itself gets transformed
into one that is capable of allowing an embryo to implant.
To determine its thickness appropriately, the uterine lining should be measured before being
exposed to progesterone at the very end of the proliferative phase.
This is an ultrasound image of the uterus.The outer portion is muscle. The inner portion is
the uterine lining. The dividing line between the two are these white lines.
The line in the middle is the uterine cavity. Remember, that normally,
the cavity of the uterus is not an open space but only a potential space where the uterine
lining from opposite sides meet. Think of your tongue pressed up against the rood of your mouth.
When we measure the uterine lining,
we are actually measuring the thickness of the upper and the lower layer added together.
What do scientific studies say about the importance of the uterine lining thickness?
One of the largest studies was performed in Canada from 2013 to 2015. They looked at over
14,000 frozen embryo transfers with day 5 or 6 blastocysts. The ongoing pregnancy rates were
no different if the lining was 8 mm or thicker, between 7 and 8 mm or between 6 and 7 mm. However,
the rate of pregnancy dropped between 5 and 6 mm. You might think that below 6
mm is a cutoff. But then the pregnancy rate in women whose lining was between
4 and 5 mm was no different than in the thicker lining groups.
There are some limitations of this study. First, there were only a very small number
of women who had embryos transferred when the lining was less than 6 mm.
In fact, less than 1% of the embryo transfers analyzed were less than 6 mm.
Second, these data were from 40 different IVF centers. It is possible that the sonographers
and the ultrasound machines might have been slightly different from center to center.
In other words, a 5 mm lining is one center could measure at a 6 mm linen an another.