Bottom Line Up Front
The endometrial lining must be both thick enough (≥7mm) and receptive at the right time for successful implantation. The "window of implantation" — when the lining is ready to receive an embryo — lasts approximately 12–24 hours. ERA testing can personalize transfer timing for patients with repeated implantation failure. Thin or non-receptive lining is a treatable condition.
What Makes a Lining "Ready"
A receptive endometrium has three key characteristics: adequate thickness (≥7mm, ideally 8–12mm on ultrasound), a triple-line (trilaminar) pattern on ultrasound indicating proper development, and the right molecular markers (integrins, cytokines, pinopodes) that signal readiness to accept an embryo.
The first two are visible on ultrasound during monitoring. The third requires specialized testing.
The Window of Implantation
In a natural cycle, the window of implantation opens approximately 7 days after the LH surge (around day 20 of a 28-day cycle) and lasts roughly 12–24 hours. During medicated frozen embryo transfer cycles, this window is determined by the number of days of progesterone exposure — typically transfer occurs on day 5 of progesterone for a Day 5 blastocyst.
For most patients, this standard timing is correct. But in approximately 25–30% of patients with recurrent implantation failure, the window may be shifted — the lining becomes receptive earlier or later than expected.
ERA Testing: Personalizing Transfer Timing
The Endometrial Receptivity Analysis (ERA) is a biopsy-based test that evaluates genetic expression in the lining to determine whether it's pre-receptive, receptive, or post-receptive at the time of sampling. If the window is displaced, your RE adjusts progesterone timing accordingly — a personalized embryo transfer (pET).
ERA testing is typically recommended after 2+ failed transfers of chromosomally normal embryos. It adds one additional cycle (the biopsy mock cycle) and costs $800–$1,500. For patients with displaced windows, the adjusted timing can make a meaningful difference.
When Lining Is Thin
A lining consistently below 7mm can reduce implantation rates. Interventions that your RE may consider include extended estrogen supplementation (higher doses or longer duration), vaginal sildenafil (Viagra — promotes endometrial blood flow), pentoxifylline and vitamin E (improve endometrial perfusion), granulocyte colony-stimulating factor (G-CSF) intrauterine infusion (emerging evidence), and platelet-rich plasma (PRP) intrauterine infusion (experimental but promising). In some cases, no intervention adequately thickens the lining, and alternative options (gestational carrier) may be discussed.
Important Context
Lining thickness is measured at its maximum (across both layers). Some ultrasound reports measure single-layer thickness, which should be doubled. Make sure you and your clinic are referencing the same measurement.