In IVF-ET, controlled ovarian stimulation with gonadotrophins and a GnRH antagonist protocol is used. Which correctly describes the 'progestin-primed ovarian stimulation (PPOS)' protocol and its main advantage?

• A Progestins are given after oocyte retrieval to support the luteal phase and reduce OHSS
• B Oral progestins are given with gonadotrophins to prevent LH surge, allowing flexible timing of oocyte retrieval without a GnRH agonist or antagonist ✓
• C Progestins are given to downregulate GnRH receptors before stimulation begins, similar to a long agonist protocol
• D Progestins suppress premature ovulation but cause higher FSH requirements and more cancelled cycles

Explanation

In the PPOS protocol, oral progestins (medroxyprogesterone acetate or dydrogesterone) are co-administered with exogenous gonadotrophins starting from day 2-3. Progestins suppress the LH surge by acting on progesterone receptors in the hypothalamus/pituitary, effectively preventing premature luteinization without the need for expensive GnRH analogues. Its main advantage is cost-effectiveness (particularly relevant in China where the protocol was developed) and flexible scheduling. Because any progesterone elevation during stimulation impairs endometrial receptivity, a freeze-all strategy with frozen-thawed embryo transfer (FET) is mandatory in PPOS cycles. This is particularly useful for high-risk OHSS patients requiring segmented IVF.

Reference: Shaw's Textbook of Gynaecology, 17th ed.

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