FET Due Date Calculator: How I Worked Out My Exact Date

Getting a due date after IVF or a Frozen Embryo Transfer feels very different from a natural pregnancy. With IVF and FET we know the exact day the embryo went back in, and often the exact age of that embryo. This is powerful information. 

It lets us calculate a very accurate estimated due date without guessing when we ovulated. In this guide I will explain how an IVF and FET due date calculator works, how you can do the maths yourself in seconds, why embryo day matters, and how I personally did it with my own transfer dates. 

I will also answer the most common questions women ask in support groups, like whether ICSI changes the due date or why day 5, day 6 and day 7 blastocysts end up with the same calculation window.

Quick Takeaway For Busy Readers

• For day 3 transfer: transfer date + 263 days = estimated due date
• For day 5 transfer: transfer date + 261 days = estimated due date
• For day 6 transfer: transfer date + 260 days = estimated due date
• General rule: Due date = transfer date + (266 − embryo age in days)

These numbers come from the biological truth that a full term pregnancy is about 266 days from fertilisation. Since the embryo already lived in the lab for a few days, we subtract that age from 266 and then add what is left to the transfer date.

What an IVF and FET Due Date Calculator Actually Does

An IVF or FET due date calculator asks for two pieces of data:

1. Your embryo transfer date
2. Your embryo age on transfer day (for example, day 3 cleavage stage or day 5 or day 6 blastocyst)

It then adds the right number of days to give you your estimated due date. You do not need to guess ovulation or use the last menstrual period like in natural conception calculators. The calculator uses fertilisation timing instead, which is more precise in IVF.

My Real Timeline So You Can Copy The Steps

Here is how I personally calculated my due date after FET.

• Transfer date: 15 January 2025
• Embryo stage: day 5 blastocyst
• Formula: 266 minus 5 equals 261 days to add
• Due date: 15 January 2025 plus 261 days equals 3 October 2025

I then went to my early pregnancy ultrasound at 8 weeks and the crown rump length matched this date, so my obstetrician kept 3 October 2025 as my official estimated due date.

I also ran a second scenario for a friend who had a day 3 transfer on 5 March 2025. We did 5 March plus 263 days and got 23 November 2025. Her clinic calculator showed the same. That gave her confidence before the first scan.

Step By Step: How You Can Do It in Seconds

1. Write down your transfer date.
2. Confirm the embryo age at transfer: day 3, day 5, day 6 or something else. Your clinic usually tells you.
3. Apply the rule: Due date = transfer date + (266 − embryo age).
4. Use any free date calculator on your phone to add the days.
5. Book your first trimester ultrasound. Your doctor will usually keep the same due date unless the measurements are very different, which is rare in IVF.

Why Day 5, Day 6 and Day 7 FETs Often Give the Same Window

Embryos transferred on day 5, 6 or 7 are all at the blastocyst stage. They are only one or two days apart in age. The difference between adding 261 days, 260 days or 259 days is small. Clinically, the estimated due dates often fall within the same week. The embryo stage mainly matters to make the calculator precise, but those one or two days do not usually change your prenatal test scheduling.

Does ICSI Change Your Due Date

No. ICSI helps a sperm enter the egg. It does not change how we count the pregnancy. Your due date still depends on the age of the embryo on transfer day and the transfer date itself.

Fresh IVF Versus FET Calculations: What Changes

Fresh IVF cycle

• Eggs are retrieved and fertilised in the lab.
• Embryos are transferred on day 3 or day 5 of the same cycle.
• You can date pregnancy either from egg retrieval plus 266 days, or more commonly from transfer date plus 263 days (day 3) or 261 days (day 5).

FET cycle

• Embryos were frozen earlier and transferred in a later cycle.
• You date pregnancy from transfer date plus the correct number for embryo age as above.
• The day you had egg retrieval months earlier does not matter for dating the FET pregnancy.

Why IVF and FET Calculators Are So Useful For Women

1. They Reduce Confusion Quickly

You do not need to fit into the last period model that natural conception calculators use. IVF gives you precise lab timings. A calculator embraces that.

2. They Help Plan Your Prenatal Tests

NIPT, nuchal translucency scan, and the anatomy scan have ideal timing windows. An accurate due date lets you schedule them correctly.

3. They Support Mental Calm

IVF pregnancies can be emotionally loaded. Having exact numbers feels grounding. I printed mine and stuck it on the fridge. It helped me believe it was real.

4. They Are More Accurate Than Natural Dating

Because fertilisation timing is known, IVF dates are less likely to need big changes later. Your early ultrasound usually confirms the calculator.

Are IVF or FET Babies Early or Late

Every baby chooses their own birthday, but there are patterns. Many studies show fresh IVF pregnancies sometimes deliver a little earlier, often between 37 and 39 weeks. FET pregnancies tend to have slightly longer gestations on average, often around 39 weeks and sometimes higher birth weights. This does not mean you will be early or late. It just tells you what is common in groups. Your doctor will individualise your plan based on your history, your placenta, your blood pressure and other factors.

Does the Due Date Ever Change After You Calculate It

In most IVF and FET cases the due date set by the fertility clinic remains the official date. Early ultrasounds tend to confirm it because the embryo age is known. Very small adjustments can happen if the crown rump length is significantly off the expected size, but that is uncommon. Still, go to all scans and appointments so you are monitored properly.

How I Used My Calculated Date to Guide My Pregnancy

This is how I used my final date to feel in control:

• I put the due date into my phone and set weekly reminders that said things like 11 weeks 2 days today.
• I used the date to schedule my NIPT blood draw during the right week.
• I counted my progesterone and estrogen stop dates in a medicated FET to the calendar, which gave me a psychological finish line.
• I packed my hospital bag by 34 weeks and set leave dates at work with confidence.

Frequently Asked Questions

If I had two embryos transferred, how do I date it?

Use the same transfer date and embryo stage. The due date formula does not change. If both implant you will carry twins, and twins are often delivered earlier, but the official due date stays the same.

My clinic told me day 5, but the online calculator did not ask for embryo age. Did I do it wrong?

Some calculators assume day 5 because that is most common. If yours did not ask, check its help page. If you are day 3, use a tool that lets you select the stage.

I had a natural cycle FET with no medications. Does that change the math?

No. Whether your lining was prepared naturally or with estrogen and progesterone does not affect the due date equation.

Can I estimate my current week and day from the transfer?

Yes. Take your due date from the calculator. Count back in weeks to today. Or use an IVF specific pregnancy app that lets you input transfer and embryo age.

Why does my pregnancy app show a different week than my clinic?

Some general apps use last period logic. Pick an app that allows IVF or FET mode. If in doubt always follow your clinic dates.

Final Thoughts

A FET due date calculator is one of the simplest and most reassuring tools you can use after a long and emotional fertility journey. It respects the reality that your pregnancy started in a lab dish and that your embryo already had a known age when it met your uterus. 

The maths is short and sweet. The result is powerful. It helps you plan your scans, your leave, your nursery and your heart. I did it on day one after my positive beta. When the early scan matched the number, I finally let myself breathe.