Frozen Embryo Transfer (FET) Due Date Calculator

FET by the Numbers

What is a Frozen Embryo Transfer (FET)?

A frozen embryo transfer is an IVF procedure where embryos that were previously created and cryopreserved (frozen) are thawed and transferred to the uterus. FET has become increasingly popular and now accounts for the majority of IVF transfers in many clinics.

There are several reasons why embryos might be frozen before transfer:

• Freeze-all cycles: Some clinics recommend freezing all embryos to allow the body to recover from ovarian stimulation before transfer
• OHSS prevention: If there's a risk of ovarian hyperstimulation syndrome, embryos are frozen to avoid pregnancy during this condition
• Preimplantation genetic testing (PGT): Embryos are frozen while awaiting genetic testing results
• Extra embryos: Embryos remaining after a successful fresh transfer are frozen for future use
• Fertility preservation: Embryos may be frozen for years before someone is ready to become pregnant

The good news is that advances in vitrification (a rapid freezing technique) have made frozen embryo survival rates excellent, often exceeding 95%. And importantly for due date calculations, the freezing process doesn't change how we calculate your pregnancy dates.

The FET Due Date Formula

Here's the key point that confuses many patients: your FET due date is calculated from your transfer date, not your egg retrieval date.

Even if your embryos were created years ago, what matters for dating purposes is:

1. The date the embryo was transferred to your uterus
2. The age of the embryo at the time of transfer (day 3, day 5, or day 6)

The formula is identical to fresh transfers:

Most frozen embryo transfers involve day 5 blastocysts, as embryos are typically grown to this stage before freezing due to better survival rates and selection capabilities.

Step-by-Step FET Due Date Calculation

Example: Day 5 Frozen Embryo Transfer

Let's walk through a real example. Sarah had her eggs retrieved in January 2025. The embryos were frozen after reaching the blastocyst stage on day 5. She had her frozen embryo transfer on June 15, 2026.

To calculate Sarah's due date:

1. Start with the FET transfer date: June 15, 2026
2. The embryo was a day 5 blastocyst, so add 261 days
3. June 15 + 261 days = March 3, 2027

Notice that the original egg retrieval date (January 2025) doesn't factor into the calculation at all. The embryo's "biological clock" was paused during freezing, and for dating purposes, we treat it as if it just reached day 5 of development on the transfer date.

Calculating Your Theoretical LMP

Medical forms and pregnancy apps often ask for your "last menstrual period" (LMP). For FET patients, you need to calculate a theoretical LMP that matches your gestational age:

Theoretical LMP = Transfer Date - Gestational Age at Transfer

For Sarah's example:

• Transfer date: June 15, 2026
• Gestational age at transfer: 19 days (for a day 5 embryo)
• Theoretical LMP: June 15 - 19 days = May 27, 2026

If Sarah enters May 27, 2026 as her LMP in a pregnancy app, it will calculate her due date correctly as March 3, 2027.

Why the Retrieval Date Doesn't Matter

This is the most confusing aspect for FET patients. If your eggs were retrieved two years ago, it feels strange that this date is irrelevant to your pregnancy dating. Here's why it works this way:

Embryo Development Pauses During Freezing

When embryos are vitrified (frozen), all biological processes stop completely. The embryo doesn't age, develop, or change in any way while frozen. Whether an embryo is frozen for two weeks or ten years, it emerges from thawing at exactly the same developmental stage it was when frozen.

A day 5 blastocyst frozen in 2020 and thawed in 2026 is still developmentally a day 5 embryo. It will implant, grow, and develop on the same timeline as a fresh day 5 embryo created and transferred immediately.

Medical Dating is About Developmental Stage

Gestational age is really measuring the embryo's developmental progress, not calendar time. All the pregnancy milestones, growth charts, and medical guidelines are based on how developed the fetus is, not when conception occurred on the calendar.

By using the transfer date and embryo age, we're ensuring that your pregnancy is dated according to actual embryonic development, which is what matters for medical care.

FET Transfer Date vs. Egg Retrieval Date: A Comparison

Let's look at two patients with the same egg retrieval date but different transfer timelines:

Patient A: Fresh Transfer

• Egg retrieval: March 1, 2026
• Day 5 fresh transfer: March 6, 2026
• Due date: March 6 + 261 = November 22, 2026

Patient B: Frozen Transfer

• Egg retrieval: March 1, 2026 (same as Patient A)
• Embryos frozen on day 5
• FET of day 5 blastocyst: September 15, 2026
• Due date: September 15 + 261 = June 4, 2027

Even though both patients had eggs retrieved on the same day, their due dates are more than six months apart because Patient B's embryo was frozen for several months before transfer.

Medicated vs. Natural FET Cycles

FET can be performed using different protocols. The table below compares the three main approaches:

Important: The FET protocol (medicated, natural, or modified natural) does not change the due date calculation. Regardless of how your cycle was prepared, you still use the transfer date and embryo age for dating.

FET vs. Fresh Transfer Trends Over the Years

Frozen embryo transfers have grown dramatically as vitrification technology improved. Here is the approximate percentage of all IVF transfers that were FET by year:

FET vs. Fresh Transfer: Key Differences

Common FET Due Date Questions

My embryo was frozen on day 5 but I'm told it's a "day 6" transfer. Why?

This can happen if an embryo was frozen on day 5 but needed to be thawed and observed for a day before transfer, making it technically day 6 at the time of transfer. In this case, use day 6 calculations (add 260 days). Always confirm with your clinic what the embryo's age was at the actual moment of transfer.

What if my embryo was frozen on day 6?

Some embryos take an extra day to reach the blastocyst stage and are frozen on day 6 instead of day 5. If this embryo is later transferred as a day 6 blastocyst, use the day 6 calculation (transfer date + 260 days).

Does FET have a different success rate that affects dating?

FET success rates are comparable to (and sometimes higher than) fresh transfers. This has no impact on due date calculation. A successful FET pregnancy develops on the same timeline as a fresh transfer pregnancy.

I had a FET but can't remember the embryo age. What should I do?

Contact your fertility clinic—they keep detailed records of every embryo's developmental stage at the time of freezing and transfer. This information is essential for accurate dating and should be in your transfer report.

Why do some online calculators ask for my retrieval date?

Some calculators designed for fresh IVF transfers ask for retrieval date because they calculate: retrieval date + 5 days = day 5 transfer date. For FET, ignore this and enter your actual transfer date, or use a calculator specifically designed for FET (like our IVF Due Date Calculator, which handles both fresh and frozen transfers).

FET Due Date and Early Pregnancy Monitoring

After a frozen embryo transfer, you'll have specific monitoring milestones:

Beta hCG Blood Tests

Typically performed 9-12 days after transfer. Your clinic will check for a positive result and may repeat the test to ensure hCG levels are rising appropriately.

First Ultrasound

Usually scheduled around 6-7 weeks gestational age. For a day 5 FET, this is approximately 3.5-4.5 weeks after transfer. The ultrasound will confirm pregnancy location and may detect a heartbeat.

Viability Ultrasound

A follow-up ultrasound around 7-8 weeks confirms heartbeat and normal development. After this point, many clinics "graduate" you to a regular OB-GYN.

All these milestones are scheduled based on your gestational age calculated from your FET date.

Using Our FET Due Date Calculator

Our IVF Due Date Calculator works for both fresh and frozen embryo transfers. Here's how to use it for FET:

1. Enter your transfer date: This is the date your frozen embryo was transferred to your uterus
2. Select embryo age: Choose day 3, day 5, or day 6 based on the embryo's developmental stage at transfer
3. Select "Frozen (FET)": This doesn't change the calculation but helps you keep records
4. Click Calculate: Get your due date, current gestational age, and pregnancy milestones

The calculator will also show you your theoretical LMP, which you can use for medical forms and pregnancy apps.

Special Considerations for FET Pregnancies

Progesterone Support

Most FET pregnancies require continued progesterone supplementation (injections, vaginal suppositories, or oral medication) until 10-12 weeks gestation, when the placenta takes over hormone production. This doesn't affect your due date but is important for pregnancy support.

Multiple FET Attempts

If you've had previous unsuccessful FET cycles, only the transfer date of your successful (current) pregnancy matters for due date calculation. Previous transfer dates have no bearing on your current pregnancy dating.

Donor Egg FET

If your FET used donor eggs, the calculation remains the same. The transfer date and embryo age determine your due date, regardless of whether the eggs were your own or from a donor.

FET Success and Due Date Accuracy

FET pregnancies have excellent due date accuracy because:

• The transfer date is known precisely
• The embryo's developmental stage is documented
• There's no ambiguity about conception timing

Studies show that IVF pregnancies (including FET) have more accurate dating than natural conceptions, where the exact fertilization date is unknown.

However, remember that a due date is always an estimate. Only about 5% of babies are born on their exact due date. Most babies arrive within two weeks before or after the estimated due date.

Summary: FET Due Date Calculation

Key takeaways for calculating your frozen embryo transfer due date:

• Use your transfer date, not your egg retrieval date
• Add days based on embryo age: 263 for day 3, 261 for day 5, 260 for day 6
• Freezing duration doesn't matter—an embryo frozen for 5 years is dated the same as one frozen for 5 days
• FET protocol doesn't affect dating—medicated and natural cycles use the same formula
• Calculate theoretical LMP for medical forms: transfer date minus gestational age at transfer

For instant, accurate results, use our free FET Due Date Calculator. Simply enter your transfer date and embryo age, and get your complete pregnancy timeline with milestones.