Embryo Transfer: Fresh or Frozen Which Option Is Best?

Embryo Transfer (ET) is the final and most important step of In-Vitro Fertilisation (IVF). It involves placing one or more embryos into the woman’s uterus (womb) to achieve pregnancy.

In IVF, embryos are created outside the body in a laboratory by fertilising the eggs (oocytes) retrieved from the woman with sperm from her male partner.

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Technique

Embryo transfer is performed by visualising the cervix (the lower part of the uterus) and gently placing the embryo(s) into the uterus under ultrasound guidance.

The ideal placement point is usually 1 to 1.5 cm below the fundus (the top part of the uterus).

ET is typically a quick and painless procedure. A smooth, accurate, and atraumatic transfer plays a major role in improving IVF success rates.

How Many Embryos Should Be Transferred?

The number of embryos to transfer is often debated and must be decided carefully.

• Transferring more embryos may improve pregnancy rates, but it also increases the risk of multiple pregnancies (twins, triplets, or higher-order pregnancies). Multiple pregnancies are considered high-risk and may lead to complications for both the mother and babies.
• Transferring fewer embryos reduces the chance of multiple pregnancy and usually results in a singleton pregnancy, but may slightly reduce IVF success rates.

Because of this, doctors follow a balanced and individualised approach based on each patient’s age, medical history, and IVF background.

Example:

• In women of advanced maternal age (above 35 years in the Indian population) or those with two previous IVF failures, it may be recommended to transfer:
  • Two blastocysts (Day 5/6 embryos), or
  • Three cleavage-stage embryos (Day 3 embryos)
• For women below 35 years, especially in the first or second IVF cycle, the ideal approach is usually:
  • Single blastocyst transfer, or
  • Two cleavage-stage embryos

Fresh Versus Frozen Embryo Transfer

Fresh Embryo Transfer

A fresh embryo transfer happens in the same cycle in which:

• gonadotropin injections are used to stimulate the ovaries, and
• Eggs are retrieved from the ovaries.

The embryo transfer is done about 4 to 6 days after egg retrieval, depending on whether:

• Day 3 cleavage-stage embryos are transferred, or
• Day 5/6 blastocysts are transferred.

Frozen Embryo Transfer (FET)

A frozen embryo transfer is performed in a later menstrual cycle, not the same stimulation cycle. It takes place during the luteal phase of another cycle.

FET can be done in:

• a natural cycle, or
• a replacement cycle

Replacement Cycle

In a replacement cycle:

• Estrogen is given for about two weeks to prepare the uterine lining
• Progesterone is then added for 4 to 6 days
• After this, the frozen-thawed embryo(s) are transferred

Advantages of Frozen Embryo Transfer

Frozen embryo transfer offers several benefits:

1. Better Endometrial Environment

In frozen cycles, the endometrium (uterine lining) is usually more natural and physiological.

In contrast, in fresh IVF cycles, estradiol and sometimes progesterone levels may become supraphysiological (too high) due to ovarian stimulation, which can reduce implantation and lead to IVF failure.

2. Reduced Risk of OHSS

IVF stimulation medications aim to produce multiple follicles. Women with:

• Polycystic ovaries (PCO), and
• other high responders

have a higher risk of developing OHSS (Ovarian Hyperstimulation Syndrome). Severe OHSS can be dangerous and, in rare cases, life-threatening.

The risk increases if pregnancy occurs in the same cycle. To prevent this, fertility specialists may recommend a “freeze-all” strategy, where all embryos are frozen and transferred later.

3. Better Pregnancy Outcomes

Studies show that pregnancies from frozen embryo transfer often have improved outcomes compared to fresh transfers, including:

• lower risk of gestational hypertension
• reduced incidence of intrauterine growth restriction (IUGR)
• fewer preterm births
• healthier birth weights and better overall baby outcomes

Conclusion

Today, many fertility specialists prefer the “Freeze All” approach because it offers multiple benefits, including:

• significantly reduced OHSS risk
• improved implantation and success rates
• healthier pregnancy outcomes