Successful Pregnancy after Frozen Embryo Transfer in a Patient with Persistent Thin Endometrium: A Case Report

Thin endometrium ( < 7mm) poses a significant barrier to successful embryo implantation and is often associated with reduced pregnancy rates in Assisted Reproductive Technologies (ART). Various medical interventions have been utilized to optimize endometrial thickness, including Hormonal Replacement Therapy (HRT), ovulation induction, and assisted reproductive technologies.

A 35-year-old woman, gravida 2 and para 2, who had a 5-year history of secondary infertility, was referred to the infertility clinic at Hamad Medical Corporation. She had undergone many ART cycles, including Ovulation Induction (OI), Intrauterine Insemination (IUI), and two In Vitro Fertilization (IVF) cycles, along with one previous FET. All of these treatments yielded unsuccessful outcomes, with negative beta-hCG tests. Although the patient undertook multiple stimulation protocols, her endometrial thickness remained persistently thin, never exceeding 5mm. She also had a medical history of a laparoscopic repair of an isthmocele and a prior cesarean section complicated by major postpartum hemorrhage. Moreover, her husband's semen analysis revealed severe teratoasthenozoospermia, necessitating IVF or Intracytoplasmic Sperm Injection (ICSI). 

The patient underwent an antagonist protocol for controlled ovarian stimulation. During the Ovum pickup, 18 oocytes were collected, with 12 out of 18 fertilized. Six embryos were frozen for later use. She then continued with FET cycle with hormonal preparation, including HRT with oral and vaginal estradiol, and downregulation with Decapeptyl. Yet, her endometrial thickness remained at 4.5mm with an irregular endometrial stripe before embryo transfer. 

On February 4, 2025, a frozen embryo transfer was performed. A positive pregnancy test, with a beta-hCG level of 800 IU/L was confirmed on February 18, 2025. A follow-up ultrasound on March 4, 2025, showed two intrauterine gestational sacs, one of which was viable at 6 weeks and 4 days of gestation, with a fetal heart rate of 124 bpm. A subchorionic hematoma measuring 3x2 cm was noted, and the patient was transferred to antenatal care.

This case highlights the possibility of achieving a successful pregnancy, even in patients with persistently thin endometrium. While an endometrial thickness of at least 7mm is typically recommended for successful implantation, this case proves that pregnancy can still be achieved with careful hormonal preparation and embryo transfer techniques. 

Several studies have explored the relationship between Endometrial Thickness (EMT) and pregnancy outcomes in Frozen-Thawed Embryo Transfer (FET) cycles. Research indicates that thinner endometrial linings are linked with reduced clinical pregnancy and live birth rates. A retrospective analysis of 2,997 patients undergoing their first FET cycles revealed that patients with an EMT of ≤8 mm had significantly lower clinical pregnancy rates (33.4%) and live birth rates (23.8%) compared to those with thicker linings. After adjusting for variables such as age, body mass index, baseline follicle-stimulating hormone levels, FET protocol, and the number of embryos transferred, medium endometrial thickness (9-13 mm) was associated with higher clinical pregnancy rates (adjusted Odds Ratio [aOR]: 1.39; 95% Confidence Interval [CI]: 1.10-1.77; p < 0.01) and live birth rates (aOR: 1.50; 95% CI: 1.16-1.95; p < 0.01) [1].

Similarly, another study involving 1,111 FET cycles found that cycles with EMT >8 mm had significantly higher chemical pregnancy rates (30.3% vs. 24.6%; p = 0.046) and clinical pregnancy rates (24.0% vs. 18.6%; p = 0.036) compared to cycles with EMT ≤8 mm. Multivariate analysis identified endometrial thickness and the protocols used as significant predictors of pregnancy success, with an odds ratio of 1.54 (95% CI: 1.07-2.22; p = 0.019) for endometrial thickness [2]. 

Another study analyzing 1,627 FET cycles reported a non-linear relationship between endometrial thickness and pregnancy outcomes, with an inflection point at 10.9 mm. When EMT was less than 9.5 mm, it was positively associated with clinical pregnancy rates (odds ratio: 1.16; 95% CI: 1.07-1.25). Beyond this point, the pregnancy rate did not increase significantly [3]. These findings underscore the significance of optimal endometrial thickness in achieving productive pregnancy outcomes in FET cycles. Although thinner endometria are associated with reduced success rates, pregnancies can still occur, highlighting the need for individualized treatment and careful monitoring in patients with thin endometrial linings.

With all the numerous techniques developed, persistent thin endometrium remains a significant challenge in ART. However, with tailored management, successful pregnancies are still possible. Further research is needed to identify and explore more refined and effective strategies for improving endometrial receptivity in such cases.

Shima Elbakhit M. E. Albasha conceived and designed the study. Shima Elbakhit M. E. Albasha wrote and critically edited the paper.

This study was not funded by anyone or any organization.

The authors have no conflicts of interest.

Data sharing is not applicable to this article as no new data were created or analyzed in this study.

1. Bu Z, Wang K, Dai W, Sun Y (2016) Endometrial thickness significantly affects clinical pregnancy and live birth rates in frozen-thawed embryo transfer cycles. Gynecol Endocrinol 32: 524-528.
2. Shalom-Paz E, Atia N, Atzmon Y, Hallak M, Shrim A (2021) The effect of endometrial thickness and pattern on the success of frozen embryo transfer cycles and gestational age accuracy. Gynecol Endocrinol 37: 428-432.
3. Zhang Q, Li Z, Wang Y, Lu H, Yu Y (2022) The relationship and optimal threshold of endometrial thickness with early clinical pregnancy in frozen embryo transfer cycles. Int J Gynaecol Obstet 156: 55-63.