How does Endometrin work? Learn how this vaginal progesterone insert supports pregnancy during IVF, explained in simple terms.
If your fertility doctor prescribed Endometrin, you might be wondering: what exactly does this medication do inside my body? The short answer is that Endometrin gives your uterus the Progesterone it needs to support a pregnancy — delivered right where it's needed most.
Let's break down how it works in plain English.
To understand Endometrin, it helps to understand what Progesterone does naturally.
Every month during your menstrual cycle, after an egg is released from your ovary (ovulation), your body starts producing Progesterone. Think of Progesterone as the hormone that gets your uterus ready for a guest. It transforms the uterine lining from a thin, basic surface into a thick, nutrient-rich environment — like converting a bare room into a fully furnished nursery. This is called transforming the endometrium from a "proliferative" state to a "secretory" state.
Without enough Progesterone, the uterine lining can't properly support an embryo. Even if a fertilized egg reaches the uterus, it may not be able to implant and grow.
During IVF, the egg retrieval process removes follicles from your ovaries. These follicles are what normally produce Progesterone after ovulation. When they're removed, your body may not make enough Progesterone on its own. That's where Endometrin comes in.
Endometrin contains micronized Progesterone — the same hormone your body makes, just broken down into tiny particles for better absorption. When you insert it vaginally, it dissolves and is absorbed directly through the vaginal tissue into the uterus.
This is a key advantage of the vaginal route: the Progesterone goes straight to the target organ. It's like delivering a package directly to someone's door instead of routing it through a distribution center (which is what happens with oral Progesterone, where the hormone has to pass through your digestive system and liver first).
Endometrin begins working relatively quickly after insertion. The vaginal insert dissolves within minutes, and Progesterone is absorbed into the uterine tissue within hours. You won't "feel" it working — there's no immediate sensation that tells you Progesterone levels are rising.
What you may notice over the first few days of use are Progesterone-related effects like fatigue, bloating, or mild cramping. These are signs that the hormone is active in your system. For a full list of what to expect, see our guide on Endometrin side effects.
Your fertility clinic will monitor your Progesterone levels through blood tests to make sure you're getting enough supplementation.
Each dose of Endometrin provides Progesterone for several hours, which is why it's dosed two to three times daily. The vaginal route provides a steady, localized supply of Progesterone to the uterus rather than a single large spike.
Treatment typically starts the day after egg retrieval and continues for up to 10 weeks — through the first trimester of pregnancy, when the placenta takes over Progesterone production. Your doctor will tell you exactly when it's safe to stop.
Consistency is important. Missing doses can cause Progesterone levels to drop, which could put a pregnancy at risk. If you miss a dose, insert it as soon as you remember and get back on schedule. Don't double up.
Endometrin isn't the only way to get Progesterone during fertility treatment. Here's how it compares to the main alternatives:
Progesterone in oil is the traditional approach — a daily intramuscular injection, usually in the buttock. It's effective, but many patients find the injections painful and inconvenient. Endometrin offers a needle-free alternative with similar effectiveness for luteal phase support.
Crinone is an 8% Progesterone gel applied vaginally once or twice daily. It's another vaginal option, but some patients find the gel messier than Endometrin's solid insert. Endometrin may also provide more consistent local Progesterone levels due to its dosing schedule.
Prometrium is an oral Progesterone capsule. While convenient, oral Progesterone must pass through the digestive system and liver (first-pass metabolism), which reduces how much reaches the uterus. Some doctors prescribe Prometrium vaginally off-label to bypass this issue, but Endometrin was specifically designed for vaginal use.
The Xiromed generic, approved in September 2025, is therapeutically equivalent to Endometrin. It contains the same 100 mg of micronized Progesterone and works the same way. The main advantage is cost — the generic can be as low as $91 for 21 inserts compared to $327–$393 for brand-name Endometrin.
Endometrin works by delivering Progesterone directly to your uterus, transforming the lining into a supportive environment for embryo implantation and early pregnancy. It's a well-studied, effective medication that gives your body what the IVF process may have taken away.
If you have questions about whether Endometrin is right for your treatment, talk to your fertility specialist. And if you need to fill your prescription, Medfinder can help you find it in stock near you.
You focus on staying healthy. We'll handle the rest.
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