Clomid and Uterine Lining: Should You Worry?
Clomid is one of the most commonly used fertility medications in the world. There has been a lot of concern about Clomid and its effect on the uterine lining. Does Clomid make your lining thinner? Does that make it less likely that you will get pregnant?
The uterine lining (endometrium)
The uterine lining is a layer of cells that surrounds the uterine cavity. As an egg matures, the ovaries produce estrogen. After ovulation the ovaries also produce progesterone.
Estrogen makes the uterine lining grow thicker. Progesterone transforms the lining so that an embryo can implant and produce a pregnancy.
Measuring the uterine lining thickness
Using ultrasound your doctor can see the uterine lining and can measure how thick it gets before and even after ovulation. There have been many attempts to determine whether there is a minimum uterine lining thickness that is needed for a pregnancy to occur.
Why is there concern about Clomid affecting the uterine lining?
It has to do with how Clomid works. Clomid or clomiphene citrate is a type of medication called a selective estrogen receptor modulator. That means that it increases the effects of estrogen in some parts of the body and decreases or blocks the effect of estrogen in other parts of the body.
Women who take Clomid because they are trying to get pregnant will usually develop more eggs and therefore produce more estrogen. But the Clomid might interfere with the estrogen's ability to make the uterine lining thicker. That's the theory. But does it actually happen in real life?
Does Clomid make the uterine lining thinner?
Studies have compared the thickness of the lining with Clomid to natural cycles or compared to other fertility medications. Most studies show a slightly thinner lining with Clomid compared to natural cycles or compared to injections. But similar thickness when compared to the medication Letrozole.
The difference in thickness is not huge. Most studies found a difference of one millimeter or less. One study looked at whether doing multiple rounds of Clomid over and over would make the lining thinner each time. That did not seem to happen.
Does a thinner uterine lining reduce pregnancy rates?
Most studies showed a thinner lining with Clomid however it didn't seem to make a difference in the chance for pregnancy. Several recent large studies found that just as many pregnancies occurred in women taking Clomid who had a thicker lining as women who had a thinner lining.
Stated another way, in women taking Clomid there was no difference in the thickness of the uterine lining in women who became pregnant compared to those who did not.
Our bottom line
The uterine lining gets thicker when women use Clomid, just not as thick as it gets naturally or from fertility injections. Don't worry too much if your lining doesn't get as thick before ovulation. You have just as good a chance for becoming pregnant as Clomid users with a thicker lining.
Having a thin uterine lining is no reason to cancel a Clomid cycle.