What is the coasting phenomenon in In Vitro Fertilization (IVF) and how is it implemented to prevent Ovarian Hyperstimulation Syndrome (OHSS)?

Coasting Phenomenon in IVF

Coasting is a technique where gonadotropin injections are withheld while continuing pituitary suppression, delaying hCG trigger until estradiol levels decrease to safer thresholds, thereby preventing severe ovarian hyperstimulation syndrome (OHSS) in high-risk patients. 1

Definition and Mechanism

Coasting involves stopping exogenous gonadotropins and postponing HCG administration until serum estradiol (E2) levels decrease to predetermined safe levels, typically while maintaining GnRH agonist suppression 2. This strategy allows follicular development to plateau while reducing the hyperestrogenic state that drives OHSS pathophysiology 3.

Implementation Criteria

When to Initiate Coasting

Coasting should begin when the leading follicle reaches 15-16 mm diameter AND estradiol levels exceed specific thresholds 3, 4:

• Standard threshold: E2 >3000 pg/ml with evidence of excessive follicular response (>10 follicles per ovary) 4
• Early coasting threshold: E2 >1500 but <3000 pg/ml in high-risk patients (obese women with PCOS showing >10 follicles per ovary) 3

Monitoring During Coasting

• Daily E2 measurements are mandatory during the coasting period 5
• HCG trigger is administered when E2 falls to <3000 pg/ml (or <17,000 pmol/L depending on protocol) 4, 5
• Continue GnRH agonist throughout the coasting period to maintain pituitary suppression 3

Optimal Duration

Coasting duration should be limited to ≤3 days whenever possible to maintain optimal pregnancy outcomes 1, 4:

• Coasting for 1-3 days maintains acceptable implantation rates (26%) and clinical pregnancy rates (52%) 4
• Prolonged coasting (>3 days) significantly reduces pregnancy rates: implantation rate drops to 18% and clinical pregnancy rate to 36% 4
• Coasting ≥4 days results in fewer oocytes retrieved and decreased implantation rates, though overall pregnancy rates per transfer may remain acceptable 5

Clinical Outcomes

Effectiveness for OHSS Prevention

Coasting reduces severe OHSS incidence to approximately 1.3% in high-risk patients, though it does not eliminate the risk entirely 4, 2:

• In a large series of 1,223 patients, severe OHSS occurred in only 16 cases (1.3% of coasted patients, 0.13% of all stimulated cycles) 4
• Ascites may still occur in approximately 16% of coasted cycles, with 2.5% requiring hospitalization 2

Impact on IVF Outcomes

Fertilization rates remain acceptable (63-73%) with coasting, and pregnancy rates are maintained when coasting duration is appropriately limited 3, 4:

• Normal fertilization (73.9%) and cleavage rates (87.7%) are achieved despite gonadotropin withdrawal 3
• Clinical pregnancy rates of 45-52% are achievable with proper coasting protocols 3, 4
• Oocyte quality and embryo development are preserved when coasting is brief 5

Special Considerations

High-Risk Populations

Patients with PCOS, high follicle counts, or rapidly rising estradiol levels are ideal candidates for coasting 6:

• Obese PCOS patients with excessive follicular response benefit from early coasting protocols 3
• Patients with underlying thrombophilia require particular caution, as OHSS increases thrombotic risk 6

Alternative Strategies

When coasting alone may be insufficient, consider these complementary approaches:

• GnRH antagonist protocols with GnRH agonist trigger are superior for high-risk patients and should be preferred over coasting when feasible 1
• Freeze-all strategy prevents pregnancy-associated late-onset OHSS and should be implemented in very high-risk cases 7, 1
• Cabergoline starting from the day of HCG reduces OHSS incidence without lowering pregnancy rates 1

Common Pitfalls

• Avoid coasting beyond 3 days as this compromises endometrial receptivity and implantation rates more than oocyte quality 5
• Do not rely solely on E2 thresholds—follicle number and patient risk factors must be considered together 4
• Pregnancy will worsen OHSS—if severe risk persists despite coasting, freeze all embryos rather than proceeding with fresh transfer 7
• Monitor for late-onset OHSS even after successful coasting, particularly in patients who achieve pregnancy 3