Purpose: Comparing a GnRH agonist and an antagonist protocol for IVF/ICSI with regard to

1. frequency of ovarian hyperstimulation syndrome (OHSS) (1. outcome measure)
2. quality of life (2. outcome measure)
3. live birth rate (2. outcome measure)
4. gene expression profiles of granulosa and cumulus cells, and concentrations of estradiol and vascular endothelial growth factor in follicular fluid(not only compared between GnRH agonist and antagonist protocol, but also between patients with OHSS and no OHSS and patients becoming pregnant and not becoming pregnant (2. outcome measure), and
5. number of oocytes removed per treatment, number of embryo transfers per treatment and number of spontaneous abortions per treatment (these three parameters are tertiary outcome measures).

In addition to the above mentioned efficacy outcome measures the safety outcome measure "frequency of known side-effects" will be compared between the two protocols.

Patients: 1100 patients are randomized prospectively to either treatment. Stratification: =<36 y/>36 y, IVF/ICSI, and treatment centre.

Methods: OHSS is quantified by consecutive measurements of weight, abdominal diameter, ultrasound measurements of ascites and ovarian volume, paraclinical parameters, and by a systematic patient questionnaire. Furthermore it is registered if the patients have been hospitalized and/or have had ascites drainage performed due to OHSS. Data from the first 100 patients are used together with Golans OHSS-classification, to make a more precise definition of OHSS. This definition is applied prospectively on data from the remaining patients.

Quality of life is gauged by a questionnaire. Child birth rate is ascertained by a pregnancy response questionnaire and by obtaining data from the Danish National Birth Registry.

Statistics: The study is designed to be able to show a 50% reduction of OHSS comparing short to long protocol.

Inclusion Criteria:

• All patients referred for infertility receiving their first IVF or IVF+ICSI treatment

Exclusion Criteria:

• Previous IVF or IVF+ICSI-treatment
• Uterine anomalies
• It is necessary to perform direct sperm aspiration from husband's/partner's testicles
• Allergy to one of the intervention products
• Patient is 40 years or above