Uterine lining thickness: what helps it grow

What does a healthy lining look like on ultrasound?

When a doctor looks at your uterine lining on ultrasound, they're often checking for a trilaminar, or triple-line, pattern. A healthy trilaminar lining shows three distinct layers: a thin bright inner line, a darker middle band, and a bright outer layer. This layered look is one visual clue used to judge whether your lining is developing the way it should.

That said, the trilaminar pattern is only one piece of the picture. It's not even the most reliable one on its own. In a review of studies on a receptive, implantation-ready lining, a trilaminar pattern correctly flagged receptivity 87% of the time. That's useful. But it's usually considered together with thickness and volume measurements, not by itself.

How does your lining change through your cycle?

Your lining isn't fixed. It builds up and sheds on a monthly rhythm. During your period, it sheds down to just 1 to 4mm before it starts rebuilding. From there, it gradually thickens, reaching roughly 12 to 13mm by the time you ovulate.

This rebuilding is driven by hormones. Rising estrogen in roughly the first half of your cycle (the follicular phase) is the main driver of lining rebuilding. It triggers your lining's cells to grow and regenerate. In other words, the same estrogen rise that leads up to ovulation is also doing the work of regrowing your lining from its lowest point after your period.

Why does 7mm keep coming up?

If you've seen the number 7mm mentioned about your lining, there's a reason doctors focus on it. A lining thicker than 7mm is a highly reliable ultrasound sign of a receptive, implantation-ready lining. It correctly identifies receptivity 99% of the time in a review of studies. For comparison, lining volume over 2mL was accurate 93% of the time, and the trilaminar pattern was accurate 87% of the time. That's part of why thickness is usually the headline number.

On the other end, doctors generally call a lining clinically "thin" when it measures 7mm or less during the mid-luteal phase. This is roughly the second half of your cycle, before your period. Timing matters here: the window when an embryo can implant falls around cycle days 20 to 24 in a natural cycle. That's close to when this mid-luteal measurement is taken, since that's when the lining actually needs to be ready.

What can stop a lining from building up properly?

For some people, a thin lining isn't just about needing more time or more estrogen. It can come from physical scarring inside the uterus. Scarring from procedures like dilation and curettage (D&C, a procedure that removes tissue from the uterus) is a well-documented cause of a damaged lining that has trouble rebuilding. This scarring most often follows a D&C done after an incomplete miscarriage, heavy bleeding after birth, or an elective abortion.

If you've had one of these procedures and have noticed your lining consistently measuring thin, it's worth raising with your doctor. Scar tissue behaves differently from healthy lining tissue. It's a separate issue from a lining that just needs more time or hormone support to build.

What actually helps your lining grow?

Rising estrogen is the main driver of lining regrowth in roughly the first half of your cycle. Because of this, hormone support and timing are central to how doctors typically approach a thin lining. This is also why lining checks happen at specific points in your cycle. Measuring earlier in your cycle versus the mid-luteal phase tells your doctor very different things about where your lining is in its rebuilding process.

It's also worth knowing what doesn't directly thicken the lining, even though it might still help. Low-dose aspirin doesn't directly thicken your lining. But research suggests it may still help pregnancy outcomes by improving blood flow to the lining and its overall tissue quality. That's an important distinction: thickness on a scan is one measure of a healthy lining. But blood flow and tissue quality matter too, and they're not always visible in a single number. Any change to your routine, including aspirin, is worth discussing with your doctor first rather than starting on your own. It works differently, and your own history matters.

Frequently asked questions

Sources

1. On ultrasound, a healthy trilaminar (triple-line) endometrium appears as three distinct layers: a thin bright inner line, a darker middle band, and a bright outer basal layer. — StatPearls/NCBI (Proliferative and Follicular Phases of the Menstrual Cycle)
2. The uterine lining is thinnest during a period, measuring only about 1 to 4 mm, and builds back up to roughly 12 to 13 mm by the time of ovulation in the proliferative phase. — Cleveland Clinic
3. During menstruation the endometrium sheds down to just 1 to 4 mm thick before it starts rebuilding again. — Cleveland Clinic
4. An endometrial thickness greater than 7 mm is a highly sensitive ultrasound marker of a receptive, implantation-ready lining, correctly flagging receptivity 99% of the time in a meta-analysis, compared with 93% for endometrial volume over 2 mL and 87% for a trilaminar pattern. — StatPearls/NCBI (Endometrial Receptivity)
5. The window during which an embryo can implant into the endometrium falls around cycle days 20 to 24 in a natural menstrual cycle. — StatPearls/NCBI (Endometrial Receptivity)
6. Doctors generally classify an endometrium as clinically 'thin' when it measures 7 mm or less during the mid-luteal phase of the cycle. — PMC review (New advances in the treatment of thin endometrium)
7. Rising estrogen during the follicular phase is the key hormonal driver of endometrial rebuilding, acting on estrogen receptors to trigger proliferation and regeneration of the endometrial lining's cells. — PMC review (New advances in the treatment of thin endometrium)
8. Low-dose aspirin doesn't directly thicken the endometrium, but research suggests it may still help pregnancy outcomes by improving the lining's blood flow (perfusion) and overall tissue quality. — PMC review (New advances in the treatment of thin endometrium)
9. Uterine scarring from procedures like dilation and curettage (D&C) is a well-documented cause of a damaged, poorly-building endometrium: intrauterine adhesions most often follow pregnancy-related curettage after incomplete miscarriage, postpartum hemorrhage, or elective abortion. — PMC (Diagnosis and Management of Asherman's Syndrome)