Pituitary down regulation is widely used during in vitro fertilization (IVF) procedures. It can suppress the spontaneous luteinizing hormone (LH) surge, and can also make follicles growth evenly.

At present, standard gonadotropin releasing hormone analogue (GnRH-a) long protocol is used in arou Long term GnRH-a, which is usually used in patients with endometriosis,has been shown to increase IVF outcome by altering endometrial receptivity. Moreover, it is also very convenient because patients do not have to inject GnRH-a daily. In order to make patients feel more comfortable during IVF treatment, the investigators have used a new ovarian stimulation protocol since the beginning of 2015. The investigators give patients long term GnRH-a (3.75mg) once at menstrual cycle D2, and begin the use of Gn to stimulate follicle growth around 30 days later based on the hormone levels and follicle size. In the pilot observational study of several patients, no LH surge occurred during ovarian stimulation.

The aim of this randomized controlled study is to compare the efficiency of this new protocol and standard GnRH-a long protocol in the investigator's center.

The primary outcome is clinical prengancy rate. Secondary outcome is cumulative live birth rate. Other parameters include number of oocytes retrieved, 2 pronucleus (2PN) fertilization rate, high quality embryo formation rate, implantation rate, spontaneous abortion rate, and ectopic pregnancy rate.

Inclusion Criteria:

• First IVF cycle;
• With normal ovarian reserve(FSH < 10 mIU/mL; antral follicle count>5);
• With normal uterine.
• PCOS women according to Rotterdam criteria

Exclusion Criteria:

• Pre-implantation genetic diagnosis cycles;
• Oocyte donation or sperm donation cycles.