One of the complications every donor should understand before starting treatment is ovarian hyperstimulation syndrome, or OHSS.

OHSS happens when the ovaries respond more strongly than expected to the medicines used to stimulate follicle growth. Most people who undergo ovarian stimulation do not develop serious OHSS, and modern treatment protocols are designed to reduce the risk. Still, knowing the warning signs matters.

What is OHSS?

During an egg donation cycle, fertility medication encourages several follicles to grow at the same time so that multiple eggs may be collected.

In OHSS, the ovaries become enlarged and fluid can move from the bloodstream into the abdomen and, in more serious cases, around the lungs. Symptoms usually develop after the final maturation injection or after egg collection.

What symptoms should I look out for?

Mild symptoms can include:

• bloating or abdominal fullness

• mild abdominal discomfort

• nausea

• temporary weight gain

Contact your clinic urgently if symptoms become more significant, especially if you develop:

• worsening or severe abdominal pain

• rapid weight gain or marked swelling

• persistent vomiting

• reduced urine output

• shortness of breath

• dizziness or fainting

Severe OHSS is uncommon but can become a medical emergency.

How common is severe OHSS in egg donors?

The HFEA describes egg donation as generally very safe. In its corrected 2024/25 UK fertility-sector data, only one severe or critical OHSS incident involved an egg donor, from 68 severe or critical OHSS cases reported across UK licensed fertility clinics that year.

That does not mean the risk is zero. It shows why donor screening, individualised medication doses and close monitoring are essential throughout stimulation.

Why are some donors at higher risk?

The chance of an excessive ovarian response varies from person to person. Your clinic will assess factors such as your ovarian reserve, ultrasound findings, previous response to stimulation and the number of follicles developing during treatment.

A strong ovarian response is not automatically a problem. The important issue is whether the response is becoming unsafe.

How do clinics reduce the risk?

A fertility team may reduce OHSS risk by:

• choosing a stimulation dose based on your individual assessment

• monitoring follicle growth with ultrasound and, where appropriate, blood tests

• adjusting medication if your ovaries are responding too strongly

• using an appropriate final maturation or “trigger” injection

• cancelling or changing the plan if continuing would place you at unnecessary risk

A donor cycle should never prioritise the number of eggs collected over the donor's safety.

Does OHSS affect future fertility?

For most donors, ovarian stimulation and egg donation do not appear to reduce future fertility. Serious complications are uncommon, but any severe medical complication deserves prompt treatment and follow-up.

Current evidence on the very long-term health effects of egg donation remains limited, which is why informed consent and responsible limits on repeat stimulation are important.

Questions to ask before starting your donor cycle

• What is my individual risk of OHSS?

• How will you monitor my response to the medication?

• What symptoms should I report immediately?

• Who do I contact after hours?

• Could my dose or trigger injection be changed if I respond strongly?

• What happens if the safest decision is to cancel the cycle?

The bottom line

OHSS is an important risk to understand, but serious cases in egg donors are uncommon. Good donor care means careful screening, personalised stimulation, active monitoring and a clear plan for what happens if your ovaries respond more strongly than expected.

You should always feel able to ask why a medication is being prescribed, what your scan results mean and whether continuing the cycle is still safe for you.

Speak with Oocyt

Considering egg donation or already going through screening? Oocyt can help you understand the process and the questions to ask your fertility clinic.

Sources

• Intend Egg Donors. Understanding OHSS: Risks and Considerations for Egg Donors (2024).

• Human Fertilisation and Embryology Authority (HFEA). Egg donation: a factsheet.

• Human Fertilisation and Embryology Authority (HFEA). Risks of fertility treatment.

• Human Fertilisation and Embryology Authority (HFEA). Written evidence for the Women and Equalities Committee inquiry into egg donation and freezing (2026).