Understanding Estradiol (E2) Numbers in Egg Freezing

If you’re preparing to freeze your eggs, you’ll be learning all about female fertility hormones, including estradiol. It’s one of the key hormones your medical team tracks throughout the process, and it plays a major role in how your body responds to the medications used during ovarian stimulation. 

This article breaks down what estradiol actually is, when and why it’s tested during your cycle, and how doctors use those numbers to make decisions along the way. We’ll also cover what different estradiol levels might suggest about how your ovaries are responding, what those levels tend to look like, and what to know if your levels come back higher or lower than expected. Whether you’re just starting to explore egg freezing or already in cycle, understanding estradiol can help you feel more confident in the process.

What is Estradiol?

Estradiol (E2) is the primary form of estrogen, the main female sex hormone. It is produced by the ovaries, specifically by the granulosa cells of growing ovarian follicles, and plays a central role in the menstrual cycle. 

Estradiol helps regulate the cycle and prepares the uterus for pregnancy by promoting the growth of the endometrial lining. In a normal menstrual cycle, estradiol levels rise during the follicular phase and peak just before ovulation, signaling that a mature egg is ready to be released. 

In an egg freezing cycle, injectable medications are used to stimulate the ovaries to mature multiple follicles simultaneously, and these growing follicles produce estradiol. So when your doctor checks your estradiol levels during a cycle, they’re using it as a real-time snapshot of how your ovaries are responding.

Each follicle contributes to your overall estradiol level. More follicles usually mean higher estradiol, which is why this hormone is so helpful for estimating how many eggs might be maturing. It doesn’t replace ultrasound (your team will still count and measure follicles via imaging), but together, these tools give a more complete picture of how your cycle is progressing.

Normal estradiol numbers during an egg freezing cycle

Estradiol is measured in a simple blood test throughout your monitoring appointments. Along with ultrasound measurements of follicle count and size (AFC), estradiol tracking helps the medical team gauge your response. These numbers can vary significantly depending on the number of follicles growing and how your body responds to the medications.

A note on letrozole and E2 levels: If your doctor is using letrozole (an aromatase inhibitor) as part of your stimulation protocol, be aware that this medication intentionally lowers estradiol levels in the bloodstream by blocking the conversion of androgens to estrogen. Because of this, your E2 levels may be significantly lower than the standard ranges described in this article—even if your follicles are growing well and your cycle is progressing normally. In these cases, estradiol is still monitored, but ultrasound findings take on even greater importance in tracking your response. If you’re taking letrozole, your care team will interpret your E2 values within that context.

Baseline: under <60-80 pg/mL

You’ll typically have your estradiol tested multiple times during your cycle, starting with a baseline blood test on day 2 or 3 of your period. At that point, your estradiol should be relatively low, usually under 50–60 pg/mL. A low baseline tells your doctor that your ovaries are quiet and ready to begin stimulation. If your baseline estradiol level is elevated, it may suggest a residual cyst from a previous cycle or indicate lower ovarian reserve. In those cases, your fertility doctor may postpone the cycle or monitor more closely.

Early stimulation: <300-500 pg/mL

As stimulation begins, you’ll take injections of hormones like FSH (follicle-stimulating hormone), which encourage your ovaries to mature more follicles. Clinics usually monitor estradiol every few days, especially around days 5, 8, and 10, as they adjust medication doses and decide when to schedule your trigger shot. 

Early in the cycle, E2 levels are very low, but as the follicles develop, estradiol rises exponentially because each growing follicle secretes estrogen.

Mid-cycle: under 500-1,000 pg/mL

By mid-cycle, estradiol increases to 500-1,000 pg/mL, reflecting the growth of multiple follicles (each follicle adds to the total estrogen output).

This rapid increase is an indirect measure of ovarian response – a stronger response means more follicles producing estrogen, leading to higher E2 levels. Doctors typically check estradiol and follicle measurements around the middle of stimulation (e.g. day 5 and day 8) to adjust medication doses if needed, and then more frequently as you approach the “trigger” day, to ensure estradiol (and follicle growth) is on track.

Trigger day: 1,500–3,000 pg/mL or higher

The final estradiol measurement is often on the day of the ovulation trigger (the injection that matures the eggs before retrieval). By this point, estradiol levels are at their peak for the cycle. 

Typical values can vary widely depending on how many follicles have grown. In a “normal” responding cycle, peak estradiol might be on the order of ~1,000–2,500 pg/mL. If you have an especially positive response (common in younger women with high ovarian reserve), E2 might exceed 3,000 pg/mL. 

In contrast, a poor or low response might only see a peak E2 in the low hundreds (<1,000 pg/mL). Clinicians use these levels in real time to guide the trigger timing and type. For instance, if estradiol is very high, indicating lots of mature follicles, they may opt for a special trigger (like a GnRH agonist trigger) to reduce the risk of ovarian hyperstimulation syndrome (OHSS). If estradiol is lower than expected, it might confirm a low-yield cycle, and the team might counsel whether to proceed to retrieval or consider canceling if the response is extremely poor. 

After the trigger shot, estradiol isn’t usually monitored immediately (because at that point the focus shifts to retrieving the eggs), but the peak level you reached is a useful summary of how well the ovaries responded to the stimulation medications.

Estradiol and egg yield: Is it predictive? 

Yes, there is a clear correlation between estradiol levels and the number of eggs retrieved. One often-cited rule of thumb is that each mature follicle contributes about 200–300 pg/mL of estradiol. So, if your estradiol is around 2,000 pg/mL on trigger day, you might expect about 8 to 10 mature follicles, and likely a similar number of eggs retrieved.

Several studies back this up. A 2021 study found that estradiol levels on trigger day strongly predicted both the number of oocytes retrieved and the number that reached maturity. Other research has found that higher estradiol levels are associated with greater egg yields and higher embryo formation rates in IVF, even across different age groups.

However, estradiol alone doesn’t tell the full story. For example, some patients may have high estradiol levels but still retrieve only a few eggs. This can happen if estradiol per egg is unusually high—something that’s been linked to poorer ovarian efficiency or lower egg quality, especially in older patients. That’s why estradiol is always interpreted alongside ultrasound findings and other hormone levels like LH and progesterone.

Most importantly, the number of follicles that measure 17mm or larger on ultrasound—the ones considered “in range”—is actually the most reliable predictor of how many mature eggs will be retrieved. Estradiol levels provide helpful supporting information, but it’s the follicle count and size that offer the clearest view of likely egg yield.

Fast vs. slow responders 

Estradiol should rise as you take stimulation meds. There’s no single “right” value mid-cycle, since protocols vary, but steady upward momentum is usually what we want. In general, doctors look for estradiol levels to roughly double every two days—a sign that your ovaries are responding appropriately to the medication.

For example, one guideline defined a “fast responder” as someone whose E2 topped 300 pg/mL by about day 5, whereas a slower responder might take until day 8 to reach 300 pg/mL. Don’t fixate on any one early number – it’s the trend that counts. 

If E2 is very slow to rise, the clinic might increase your medication dose or extend stimulation a bit. If it’s skyrocketing quickly, they might adjust doses downward or start preparing to trigger a bit sooner to avoid overshooting. Don’t be afraid to ask your clinic about your results; they will tell you if your hormone levels are as expected or if any changes are needed.

Interpreting “low” estradiol

If your estradiol never climbs into the four digits by trigger day, it may indicate a lower-yield cycle. Don’t be discouraged – quality matters too! But know that a peak E2 of, say, 600 pg/mL might translate to only a few eggs retrieved. 

In an Extend Fertility study of egg freezing patients, those with peak E2 below 1,000 pg/mL (low responders) did have fewer mature eggs and a lower maturation rate compared to higher-E2 cycles. Your doctor might have a candid conversation about whether to proceed or consider another strategy if the response is very low. 

For women under 35, true low response is less common, but it can happen and may warrant investigating underlying factors. On the flip side, remember that even a low-yield cycle can still be valuable – each egg is a chance, and younger eggs (even if few) have high pregnancy potential. Your care team will help put this in perspective based on your goals.

The bottom line

Estradiol is your friend in the egg freezing process – it’s evidence that your ovaries are doing what we want them to do. By understanding the typical patterns (low at baseline, rising through stimulation, and peaking at trigger), you can better follow along with your cycle monitoring. 

Rather than getting anxious over an isolated lab number, look at the big picture: Is your estradiol increasing appropriately? Approximately how many follicles does it suggest? Your fertility team will interpret these values with the nuance they require. High or low, the estradiol levels guide your doctors in optimizing your cycle. And as an empowered patient, knowing what estradiol signifies helps you ask informed questions. For example, if you hear your E2 value, you might now recognize “Okay, that sounds like a strong response” or “Hmm, that’s on the lower side – what does that mean for my egg count?” 

Remember that every woman’s ovaries are unique and every cycle is unique. Use these numbers as informative benchmarks, but always discuss specifics with your doctor, who can correlate estradiol with your ultrasound findings and overall plan. With an evidence-based, well-monitored approach, you can feel confident that your estradiol levels – and the precious eggs they reflect – are being managed to give you the best possible outcome for the future.

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Sources:

• Deadmond A, Koch CA, Parry JP. Ovarian Reserve Testing. [Updated 2022 Dec 21]. In: Feingold KR, Ahmed SF, Anawalt B, et al., editors. Endotext [Internet]. South Dartmouth (MA): MDText.com, Inc.; 2000-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK279058/
• Huang, W., Wei, L., Tang, J. et al. Impact of relative estradiol changes during ovarian stimulation on blastocyst formation and live birth in assisted reproductive technology. Sci Rep 15, 15617 (2025). https://doi.org/10.1038/s41598-025-00200-5
• Malathi, A., Balakrishnan, S. & B. S., L. Correlation between estradiol levels on day of HCG trigger and the number of mature follicles, number of oocytes retrieved, and the number of mature oocytes (M2) after oocyte aspiration in ICSI cycles. Middle East Fertil Soc J 26, 34 (2021). https://doi.org/10.1186/s43043-021-00080-5