Assisted reproduction step by step

The same words, different steps

IVF, ZIFT, GIFT, ICSI, AI and IUI sound alike. To tell them apart, ask three questions. Where do the gametes meet? What is placed in the woman? Where is it placed?

• IVF with ZIFT. The gametes meet in a dish in the laboratory. A zygote or an embryo of up to 8 blastomeres is placed in the fallopian tube.
• IVF with transfer into the uterus. The gametes meet in a dish in the laboratory. An embryo of more than 8 blastomeres is placed in the uterus.
• GIFT. The gametes meet in the fallopian tube. The ovum of a donor is placed in the fallopian tube.
• ICSI. One sperm is injected into the ovum in a dish. The embryo is placed in the woman as in IVF.
• AI. The gametes meet in the female tract. Semen is placed in the vagina.
• IUI. The gametes meet in the female tract. Semen is placed in the uterus.

The NEET syllabus names IVF, ZIFT and GIFT. ICSI, AI and IUI are in NCERT and are asked in other exams, but they are not in the NEET text.

A good habit is to follow the gametes. In IVF, ZIFT and ICSI the ovum and the sperm meet outside the body. In GIFT, AI and IUI they meet inside the body of the woman.

Counting chances

This part is not in the NEET syllabus, but A-Level and IB use it. A clinic may transfer more than one embryo. Suppose each embryo has the same chance of implanting, and the embryos do not affect one another. This is a made-up model for learning.

We can find every outcome by listing it. Take 2 embryos, with a chance of $\dfrac{1}{4}$ for each.

• Neither implants: $\dfrac{3}{4} \times \dfrac{3}{4} = \dfrac{9}{16}$.
• Exactly one implants: two ways, each $\dfrac{1}{4} \times \dfrac{3}{4}$, so $\dfrac{6}{16}$.
• Both implant: $\dfrac{1}{4} \times \dfrac{1}{4} = \dfrac{1}{16}$.

The three numbers add to 1. The chance of at least one pregnancy is $1 - \dfrac{9}{16} = \dfrac{7}{16}$, which is $43.75\%$. The chance of two or more is $\dfrac{1}{16}$, which is $6.25\%$.

A third embryo raises the chance of a pregnancy, but it also raises the chance of a twin or a triplet pregnancy. Such pregnancies carry more risk to the mother and the babies. This is why doctors think carefully about how many embryos to transfer.

You should find that each extra embryo raises both curves, and that the curve for two or more rises faster than the curve for at least one.

Things to think about

The technologies are costly, and they do not always succeed. A couple needs time, support and honest advice. Many countries control them by law. A couple may also choose adoption.

Try it in Python

The program lists every outcome for a number of embryos, with exact fractions. It finds the chance of at least one and of two or more.

Practise with new numbers

Common mistakes

• Adding the chances of two embryos. The chances overlap, and the sum is too large. Use one minus the chance of none.
• Mixing up ZIFT and GIFT. ZIFT places a zygote or an early embryo. GIFT places an ovum.
• Saying that fertilisation is in the body in every technology. In IVF, ZIFT and ICSI it is in a dish.
• Putting the embryo of 16 cells in the fallopian tube. The tube is for up to 8 blastomeres.
• Saying that more embryos are always better. They raise the chance of a pregnancy and also of a multiple pregnancy.

Remember these

• IVF, ZIFT and ICSI: gametes meet in a dish. GIFT, AI and IUI: they meet in the body of the woman.
• Up to 8 blastomeres: fallopian tube (ZIFT). More than 8: uterus.
• GIFT: the ovum of a donor goes into the fallopian tube.
• With independent embryos, the chance of at least one is $1 - (1 - s)^m$.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. In which techniques do the gametes meet outside the body?

2. Where is the embryo of 10 blastomeres transferred?

3. How does AI differ from IUI?

4. Two embryos each have a chance of $\dfrac{1}{4}$. What is the chance that neither implants?

5. Why may a doctor limit the number of embryos transferred?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 3, Reproductive Health.
• World Health Organization (2017). The International Glossary on Infertility and Fertility Care, Zegers-Hochschild F. et al.