Miscarriage is the loss of a pregnancy from the uterus before the
20th week. Is there a certain time at which most miscarriages occur?
What is the risk for miscarriage during every week of pregnancy? Stay tuned.
First, you need to understand how the length of a pregnancy is calculated.
Doctors use the first day of the period as the first day of the pregnancy. Medically speaking,
a pregnancy does not begin until a fertilized egg has implanted. The problem is that people
don't know when an embryo has implanted, they DO know when their last period started,
however. This is the reason why pregnancies are timed based on the last known period.
Let’s say that your period comes every 28 days. This month, because you missed your period,
you did a pregnancy test on the 28th day and it came back positive indicating you are pregnant. At
that point, you are considered 4 weeks pregnant. By day 35, you are 5 weeks pregnant and so on.
There have been a number of studies which have attempted to determine what the risk for having
a miscarriage is for every week of pregnancy. This is a lot more difficult than it sounds. First of
all, there are a lot of factors which can increase or decrease the risk for miscarriage. For example,
the older a pregnant person is, the greater the chance for the pregnancy to miscarry. Obesity also
increases the risk for miscarriage. The way that a miscarriage is diagnosed will also influence
these numbers. For example, if a pregnant person is having an ultrasound every week,
a non viable pregnancy can be diagnosed sooner than just waiting for the pregnancy to pass.
Okay, given all these limitations, I am going to give you what I feel are the most
reliable numbers that we have. I put these in a graph to make the numbers easy to understand.
You’ll notice that there are two lines. Because of all the different factors which
influence miscarriage risk, there is a range of risk for any given week. The red line is the
estimate of the higher risk and the blue line is the estimate of the lower risk. The area
between these lines would identify the weekly miscarriage rate for 95% of pregnant people.
You can notice a few things right away about this graph. The rate of miscarriage is much higher
earlier in pregnancy than it is later in pregnancy. For example, during the 4th
week of pregnancy, we would expect somewhere between 130 and 220 people would miscarry
out of every 1000 pregnancies. During the 5th week, the number of miscarriages is
between 11 and 77 per 1000. By the 15th week it is between 3 and 15 per 1000 pregnancies.
Why is the rate so high in the very early weeks of pregnancy? Abnormal embryos. Human beings produce
a tremendously high percentage of abnormal embryos. Stated another way,
each fertilized egg has only a small chance of ending up as a liveborn baby.
As a reproductive endocrinologist, I provide care to people with infertility
and people who are suffering from miscarriages. A common misconception I
hear from patients who have had multiple miscarriages is that they believe that
something is happening at a certain week of pregnancy. This is not actually what is
happening. Statistically, this is when most miscarriages occur. Here is what I mean.
A couple that I saw in my office recently had three miscarriages and
no live births. All three losses occurred between 6 and 8 weeks.
They wanted me to find out what was happening at that time that was causing them to miscarry.
Lets look at our miscarriage data again. About 25% of all miscarriages occur between 6 and 8 weeks.
That’s one in every 4. The thing is, most people who have a miscarriage before the 5th week
usually don’t even know they were pregnant. Miscarriages at this early stage are usually
mistaken to be a late period. Many more people know that they are pregnant after the 6th week.
At that point, about half of all remaining miscarriages occur between the 6th and 8th week.
Our Infertility TV bottom line is this. Miscarriages are common. The number of
miscarriages is highest early in pregnancy and decreases every week. Although some people have
a higher risk for a miscarriage compared to others, this same pattern applies to everybody.