Pregnancy versus Implantation
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Pregnancy versus Implantation
To understand this, lets back up a little and understand the difference between pregnancy
rate and implantation rate. This information is useful for both couples undergoing fertility
treatment as well as IVF clinics transferring more than two embryos.
As humans, we work towards a goal and hope to achieve it. Similarly, when you think of your
fertility treatment your wish is to get “Pregnant” and to ultimately give birth to a healthy
baby after nine months of gestation.
The term pregnancy is misleading and it is important to know the difference between
pregnancy rate (PR) and implantation rate (IR).
Through this article, we will be covering the following points:
- Pregnancy rate?
- What is implantation rate?
- Difference between the two?
- Why is it important?
Pregnancy
In a typical IVF cycle, the standard timeline is egg collection, development of embryo,
vitrifying or transfer of embryo. If, embryos are vitrified to do a frozen embryo transfer (FET).
Be it a transfer of fresh or frozen embryos, the cycle is complete only when embryos are
placed back into the uterus. The procedure of placing embryos in your uterus with the help
of a catheter is known as embryo transfer.
By now, you already know that there are two categories of embryos, those on day 2 and day
3 of embryo development are called cleavage stage. Whereas, a more advanced stage of
embryos on Day 5 are referred as Blastocyst. Depending on the stage of embryo transferred
(Cleavage or blastocyst) your clinic will ask you to test for pregnancy (B-HCG) which is ideally
14 days and 11 days after cleavage and blastocyst transfer, respectively.
It is “test day” today and you may be asked to do an easy urine pregnancy test (UPT) to
detect levels of HCG (pregnancy hormone) in your urine, which is then followed by a
confirmatory blood test to determine the quantitative level of HCG in your blood. UPT is an
indicative test whereas Beta HCG blood test gives you a definite value of the hormone.
If on the first day of your test, your UPT is positive and there is HCG in your blood.
Congratulations! you are pregnant. However, the journey to a viable pregnancy is a little
complicated and we will walk you through it. On the first day of test if your blood HCG level
is above 100 … it is a good sign, however in some case this level may rise but eventually fall.
An increase in beta HCG level with drastic dip is known as biochemical pregnancy. It
indicates that the embryo had implanted, which was detected by the rise in HCG levels. But,
due to some factors there was a loss in pregnancy, which Is accompanied by bleeding and
dip in blood HCG levels.
Pregnancy Rate
The important thing to analyze here is that you could be pregnant with a transfer of One,
two, three or sometimes even more embryos. Most IVF clinics, sadly do not concentrate on
the quality of embryos, but more on the number. They may be adopting transfer of multiple
embryos assuming “more is better”. Sadly, this mantra does not apply in embryology,
remember if the embryo is good you just need one good embryo to give you the desired
pregnancy. Whereas, on the other hand a woman may not get pregnant with a transfer of
five embryos also, if they are of poor quality. (But best practice …)
IVF clinics, advertising their “Pregnancy rates” or “success rates” are just talking about Blood
HCG positive, they do not count biochemical pregnancy, miscarriages or even how many
embryos it took for them to get this pregnancy.
So, to put this into perspective
Pregnancy rate (PR) = Number of Beta HCG positive / Number of embryo transfer * 100
Example: If clinic A has done 20 embryo transfer in the January and out of that 11 are
positive. The pregnancy rate of that clinic is 11/20 = 55%. Similarly, pregnancy rate is
calculated monthly, quarterly and yearly.
Knowing the pregnancy rate is important, but it is just the tip of the ice berg.
Implantation
Implantation is the term used to describe the process of attachment of embryo to the wall
of the uterus. The embryo needs to attach to the endometrium to grow, body starts
producing HCG after implantation which is then detected in the blood. Presence of HCG in
blood or urine renders a woman “Pregnant”. Test for HCG in blood, is done after four weeks
from your last menstrual period (LMP).
After a positive B-HCG the woman needs to undergo an ultrasound scan at 6-7 weeks. This
scan is an important milestone, where you get a glimpse of the gestational sac. This is the
first visual confirmation of your pregnancy, the number of sacs you are dependent on the
number of embryos transferred. So, if you are pregnant and you transferred one embryo,
you will see only one sac. But for example, if three embryos were transferred there is a
possibility of one, two or three gestational sacs, depending on the number of embryos
which implanted in the uterus.
To simplify things, the number of gestational Sac seen is a direct representation of the
number of embryos which have implanted and number of babies you can expect.
One embryo = one baby.
one embryo –! If pregnant —! One gestational sac –! One healthy baby.
Implantation rate:
This gives you an indication regarding how many embryos are needed to achieve one
pregnancy. This is a direct indication towards the quality of the laboratory, because you will
transfer more only if you are not confident about your embryo quality. If the clinic is
confident about their embryo grading and quality they will be able to transfer the best and
save the rest for a later cycle. Implantation rate compels you to believe and work towards
“Quality matters”
Implantation rate is calculated as
Implantation rate (IR) = Number of gestational sac / Number of embryos transferred * 100
Let’s look at few examples.
Clinic A: If clinic A has done 20 embryo transfer in January and out of that 11 are positive.
The pregnancy rate of that clinic is 11/20*100 = 55%. But if they have transferred 4 embryos
per transfer that makes the cumulative embryo transferred as 80. Now when the 11
pregnant women went from their 6-7 week scan it was observed that there were
cumulatively only 10 gestational sacs. This makes their IR as = 10/80*100 = 12.5%
Clinic B: If clinic B has done 20 embryo transfer in January and out of that 11 are positive.
The pregnancy rate of that clinic is 11/20*100 = 55%. But if they have transferred 2 embryos
per transfer that makes the cumulative embryo transferred as 40. Now when the 11
pregnant women went from their 6-7 week scan it was observed that there were
cumulatively only 10 gestational sacs. This makes their IR as = 10/40*100 = 25%
Why should we transfer a maximum of two embryos?
- If the embryo quality is good one is enough to get pregnant
- Good embryo grading ensures better embryo selection
- Transferring more is not better
- The aim should not only be to get pregnant, but to have a healthy pregnancy for 9 months thereby delivering a healthy baby
- More the number of embryos transferred, higher the risk of multiple pregnancy which is detrimental for the health of mother and baby
- Surplus embryos can be vitrified for a later cycle
- In case there is an endometrium issue, why transfer all embryos and lose them. Better to take a chance with one or two, vitrify the rest and then evaluate the situation.
The difference in implantation rate between clinic A and Clinic B is significant. Clinic A needs
up to four embryos to get a viable pregnancy. In comparison Clinic B needs just two embryos
to achieve the same. Hence, Clinic B has a better embryology program.
Implantation rate will always be lower than the pregnancy rate. As not all Beta HCG women
will have a gestational sac and more the number of embryos transferred lower the IR rate.
Clinics should have a benchmark for both PR and IR to keep a check on their quality.
Two most important terms related to pregnancy and implantation are Biochemical
pregnancy and miscarriage or missed abortion.
Biochemical pregnancy as discussed above is a condition where the BHCG levels rise initially
but later the values drop drastically. Pregnancy loss before the appearance of gestational sac
in known as biochemical pregnancy. This is counted as positive pregnancy but not as
implantation
A missed abortion or miscarriage is loss in pregnancy after the appearance of gestational
sac. This will be considered as both positive pregnancy and implantation since the loss of
pregnancy was after the appearance of gestational sac.
