I often refer to the conception process in terms of a “seed/soil” relationship. Just as a plant can’t grow and thrive without first assuring that both seed and soil are good, neither can a pregnancy be successful without both the seed (embryo) and the soil (uterine environment) being ideal. Over the next few posts, we’ll examine factors related to the “soil” half of the equation – the uterine environment. The uterine factors are as critical to this equation as is the “competency” of the embryo (the seed). When uterine factors are less than optimal, this can be manifest as “unexplained” infertility/IVF failure, early miscarriage, placental insufficiency with intrauterine growth retardation, and sometimes fetal demise.
In today’s post, I’ll discuss the physical characteristics of the uterus that can influence embryo implantation.
1. Contour of the uterine cavity: It has long been suspected that anatomical defects of the uterus might result in infertility. While the presence of fibroid tumors in the uterine wall are unlikely to cause infertility, an association between their presence and infertility has been observed in cases where they distort the uterine cavity, or protrude as submucous polyps through the endometrial lining. It would appear that even small submucosal fibroids have the potential to hinder implantation.
It is likely that any surface lesion in the uterine cavity, whether an endometrial, placental or fibroid polyp (no matter how small), or intrauterine adhesions, has the potential to interfere with implantation by producing a local inflammatory response, similar in nature to that which is caused by a foreign body such as a intrauterine contraceptive device (IUD). Unfortunately, a dye X-Ray test (hysterosalpingogram or HSG) will miss the diagnosis in approximately 20% of cases. The best methods by which to diagnose and assess even the smallest of such lesions is through the performance of a sonohysterogram (SHG), or by hysteroscopy.
2. Endometrial Thickness: In 1989, I published on the fact that in both normal and “stimulated” cycles, preovulatory endometrial thickness is predictive of embryo implantation (pregnancy) potential following IVF. Ideally, the endometrium should measure at least 9.0mm in thickness. However, healthy pregnancies can occur with linings that measure between 7.5 and 9.0mm (although much less likely). A “poor” endometrial lining is most commonly due to:
- Inflammation of the uterine lining (endometritis) that usually occurs as a result of a septic delivery, abortion or miscarriage,
- Severe adenomyosis (gross invasion of the uterine muscle by endometrial glandular tissue)
- Multiple fibroid tumors of the uterine wall
- Prenatal exposure to the synthetic hormone, diethylstilbestrol (DES)
- Following back-to-back cycles of clomiphene citrate ovulation induction.
My next post will discuss outside factors that can affect embryo implantation: Immunologic factors and the embryo transfer itself.