What is the normal size of endometrium to get pregnant?

As we know the endometrium is the uterine lining which the embryo will implant and for this to happen its thickness must be approximately 6 to 10 millimeters and have trilaminar  appearance. We can monitor endometrial thickness through an ultrasound during the menstrual cycle, controlled ovarian stimulation in assisted reproduction cycles or prior to scheduling vitrified embryo transfer.

A key player in conception or infertility in addition to the eggs, the sperm, the tubes, is the lining which unfortunately sometimes you keep on forgetting is that it’s not only creating a good embryo in the lab or a couple timing intercourse at the right time because at the end of the day, the the uterus or the endometrium (which is the lining of the uterus) it’s the place where the embryo, later on the fetus and then the baby are going to stay for nine months. The thickening of the uterus or the lining of the womb which is the endometrium is a very key factor that dictates the chances of pregnancy whether naturally or whether after fertility treatment. We measure the lining in millimeters and I always when I mention the lining there are two if not three key things about the lining. The third one is a recent advance in investigating the lining that I will touch upon at the end of my answer. The first thing is checking the thickening, so when I mentioned the lining I mentioned the thickening and the good thickness of the lining is approximately at least around seven to eight millimeter around the time of ovulation. Because similar to anything in the field of fertility what is the thickening means I have to always couple it with what day of the cycle and a good thickness of seven to eight millimeter around ovulation is considered to be fine, a little bit thicker is better, too thick is too bad, too thin is too bad. A lining that is too thick might also prevent embryos from implanting because it might be speaking about a problem in the lining (like a polyp) or some abnormal cells in the lining (like hyperplasia). Too thin of a lining is too bad, it might speak either of a lining that is not stimulated properly by hormones or it might be speaking about scarring tissue inside the lining – both of those will affect embryos to implant whether naturally or whether after fertility treatment. We mentioned about thickness of the lining but when we mentioned lining, we also mentioned the quality of the lining and whether the thickness or the lining this can be checked by doing an ultrasound scan (which is mainly an internal scan), the scan can be done through the tummy or an abdominal scan but the vaginal scans are more accurate in assessing the lining and your doctor will be measuring the thickness of the lining on a particular day and we also not only check the thickness of the lining, we check the quality of the lining and if you are reading a report about the lining, the letters A and B follow a number, so you’ll discover that the report mentions 7B, 7A or 7A/B. “B” is considered a good quality lining so a good lining is a 7B or a trilaminar lining while other linings like A or A/B are in general associated with a poorer outcome because not a good quality. One last thing I want to mention about the lining is that these days even if the lining looks perfect, a more growing field of research is targeting the implantation window in the lining correctly and that’s why these days there are some genetic testing that are done to the lining on the specific day when we put back embryos. Those studies are very promising, those studies will help us in tailoring when is the best time to put an embryo inside the lining when the lining is ready to accept that embryo.

The minimum thickness during a preparation for embryo transfer is 7 millimeters that’s known – that’s one element and the second is the morphology of the endometrium and it should be trilaminal morphology. The endometrium after an average of 12 to 14 days of preparation naturally or artificially, of7 millimeters, under trilaminar morphology is the two elements we want before going to embryo transfer.
Of course we always do our progesterone measurement before switching to the second phase so: negative progesterone, trilaminar morphology and thickness of 7 millimeters are the three basic elements to program embryo transfer.

Obviously there’s lots and lots of data that looks at the optimal thickness of the lining of the womb to get pregnant with IVF because we’re measuring it regularly for that and within that context for an IVF cycle with a fresh or a frozen embryo, we would want to see the lining to be at least seven millimeters. Some studies have looked at eight or nine but most clinics will look at seven or above and that in essence is what we want to see probably in a natural cycle as well. Again, as your egg grows, it produces estrogen. What the estrogen does is thicken up the lining of the womb, so the time to really assess the lining is probably around or just before ovulation. If you want to reassure yourself that your lining is doing what it should, if you’ve got a 28 day cycle on your way around day 14 a scan probably just before ovulation would be reassuring.

The endometrium is the lining of the uterus or the lining of the womb and having a healthy endometrium is very important in achieving pregnancy. You can imagine it’s a nest where the fertilized egg or the embryo will sit in the first few days or weeks of implantation. So a healthy endometrium is very important. Assessment of the… we call it endometrial thickness actually not the size. The thickness assesses how thick the lining of the womb is, and that would change during your menstrual cycle physiologically. So, in the first part of the cycle, gradual thickening of the endometrium, and then in the second part after ovulation, you have more thickening. If you do an ultrasound scan, you will see thickening of the endometrium around 14 or 15 millimeters, and that’s normal in the second part of the cycle.

In general, for normal conception, it’s not really important to check the endometrium. However, if we treat our patients with ovulation induction, I would like to have a nice thickened endometrium in the second part of the cycle to give me reassurance that this embryo will have a nice implantation place to sit in. So, I would love to see an endometrium thickness of more than 8 millimeters or so. This is also used in an IVF cycle. The opposite is also true if we have thin endometrium; endometrium less than 6 millimeters, it can be associated with IVF not working. There are so many causes for thin endometrium. Sometimes a condition we call uterine synechiae or adhesion or scar tissue inside the womb can lead to thin endometrium. Sometimes, there is no cause for it, it just occurs as a hormonal problem in the receptor of the lining of the womb.

The other extreme is a thickening of the endometrium can be caused by a condition we call endometrial polyp, which is certain growth of the cells of the lining of the womb. If the doctor finds this polyp, we usually do a procedure we call hysteroscopy in which they have a camera assessment and this can be removed at the same time. So yes, endometrial thickness is essential, and having healthy endometrium with a normal thickness would be associated with a good pregnancy outcome.