OHSS Risk Calculator (Ovarian Hyperstimulation)

Ovarian hyperstimulation syndrome (OHSS) is the most significant complication of controlled ovarian stimulation in IVF. Mild OHSS is common, in roughly a fifth to a third of stimulated cycles, and settles on its own. Moderate or severe OHSS is much rarer, in the low single figures per hundred cycles depending on the clinic and the patients, and involves substantial ovarian enlargement, fluid shifts, electrolyte abnormalities, and rarely blood clots or breathing problems.

Identifying high-risk patients before stimulation is what makes prevention possible. A GnRH antagonist protocol, a lower starting dose, a GnRH agonist trigger instead of hCG, and freezing every embryo for a later transfer all reduce the risk substantially, and all of them have to be decided before or during the cycle rather than after.

What this page checks, and where the numbers come from

There is no validated OHSS score. What the literature does have is a set of markers, each with a published cut-off, and the American Society for Reproductive Medicine’s guideline names PCOS, a high AMH, a high peak estradiol and many developing follicles among the factors that raise the risk. This page lists which of them apply. The thresholds are the ones collected by Fiedler and Ezcurra in 2012 (after Humaidan and colleagues), most of them from Lee and colleagues in 2008 and Papanikolaou and colleagues in 2006.

Before stimulation:

• Age under 33
• AMH (anti-Müllerian hormone) above 3.36 ng/mL, which is about 24 pmol/L
• More than 24 antral follicles across both ovaries, a polycystic-looking ovary
• A diagnosis of PCOS (polycystic ovary syndrome)
• A previous episode of moderate or severe OHSS

On the day of trigger:

• 13 or more follicles of 11 mm or larger predicts OHSS
• 18 or more of them, or an estradiol of 5,000 pg/mL or more, predicts severe OHSS

Estradiol on its own is a weak predictor. It means much more alongside the follicle count, which is why the two are read together.

Why this page does not add the markers up

An earlier version of this calculator gave each factor 1 or 2 points and turned the total into low, moderate, high and very high risk, with a treatment for each. Those weights were never published by anyone. A count of markers is honest; a points total dressed up as a risk band is not. So the page reports which markers are present and what each one is known to predict, and leaves the arithmetic of risk to the clinic, which has the patient’s full history and its own outcome data.

Low body weight is sometimes listed too, but the studies disagree, and it is not one of the guideline’s factors.

Units

US labs report AMH in ng/mL and estradiol in pg/mL. Labs in the UK, Europe and Australia often use pmol/L for both. The menus beside each box take either: 1 ng/mL of AMH is 7.14 pmol/L, and 1 pg/mL of estradiol is 3.67 pmol/L.

Limitations

This is an educational checklist, not a prediction model. Real assessment also weighs protocol history, response to prior cycles, lab kinetics through stimulation, and the clinician’s knowledge of the individual patient.

The most effective risk reduction in modern IVF is the freeze-all approach in high-risk patients. It essentially eliminates severe late-onset OHSS, because that form is driven by the hCG rise of an established pregnancy, and a frozen transfer in a later cycle removes it entirely.