Fertility specialists determine the number of embryos to transfer during in vitro fertilization (IVF) by considering various factors to balance success rates with the risk of complications. The decision-making process involves both medical guidelines and individualized assessment. Here are some key considerations:

1. Age of the Patient: Younger patients generally have a better chance of successful implantation, so they may require fewer embryos to be transferred compared to older patients.

2. Reproductive History: Previous IVF attempts, miscarriages, or successful pregnancies can influence the number of embryos to transfer.

3. Quality of Embryos: Evaluation of the quality and developmental stage of embryos plays a crucial role. Higher quality embryos are more likely to lead to successful implantation.

4. Underlying Health Conditions: Certain medical conditions in the patient may affect the decision, such as polycystic ovary syndrome (PCOS) or endometriosis.

5. Legal Regulations and Guidelines: Some countries or clinics have specific regulations regarding the number of embryos that can be transferred to reduce the risk of multiple pregnancies.

6. Personalized Risk Assessment: Fertility specialists also take into account the individual health and preferences of the patient.

The aim is to achieve a successful pregnancy while minimizing the risk of multiple pregnancies, which can pose health risks to both the mother and babies. By carefully assessing these factors, fertility specialists can make an informed decision on the optimal number of embryos to transfer during IVF.