Inclusion Criteria:

1. Be a female subject justifying an IVF/ET treatment,
2. Have a male partner with semen analysis within the past 6 months prior to randomisation considered adequate to proceed with regular insemination or ICSI according to the centre's standard practice. If these criteria are not met, the subject can only be entered if donor sperm will be used,
3. Be between her 18th and 35th birthday (35 not included) at the time of the randomisation visit,
4. A body mass index (BMI) lower than 30 kg/m2 where the BMI is calculated according to the formula.
5. Have a regular spontaneous ovulatory menstrual cycle between 21 and 35 days in length,
6. Have early follicular phase (day 2-4) serum level of basal FSH lower and equal to 12 IU/L measured in the centre's local laboratory during the screening period (i.e. within 2 months prior to down-regulation start),
7. Presence of both ovaries,
8. Normal uterine cavity, which in the investigator's opinion is compatible with pregnancy,
9. Have a negative cervical PAP test within the last 6 months prior to randomisation,
10. Have at least one wash-out cycle (defined as greater and equal to 30 days since the last dose of clomiphene citrate or gonadotrophin treatment) since the last ART cycle and/or clomiphene citrate or gonadotrophin treatment prior to starting GnRH agonist therapy,
11. Be willing and able to comply with the protocol for the duration of the trial,
12. Have given written informed consent, prior to any trial-related procedure not part of normal medical care, with the understanding that consent may be withdrawn by the subject at any time without prejudice to her future medical care

Exclusion Criteria:

1. Had greater or equal to 2 previous ART cycles with a poor response to gonadotrophin stimulation (defined as lower or equal to 5 mature follicles and/or lower or equal to 3 oocytes collected) or had greater or equal to 2 previous ART cycles with a hyper response (defined as greater or equal to 25 oocytes retrieved),
2. Any medical condition, which in the judgement of the investigator may interfere with the absorption, distribution, metabolism or excretion of the drug. In case of doubt, the subject in question should be discussed with Merck Serono's Medical responsible,
3. Had previous severe OHSS,
4. Polycystic ovary syndrome (PCOS; Rotterdam criteria) to reduce the risk of the occurrence of OHSS,
5. Presence of endometriosis requiring treatment,
6. Uterine myoma requiring treatment,
7. Any contraindication to being pregnant and/or carrying a pregnancy to term,
8. Extra-uterine pregnancy within the last 3 months prior to screening,
9. History of 3 or more miscarriages (early or late miscarriages) due to any cause,
10. Have at least one wash-out cycle (defined as greater and equal to 30 days since the last dose of clomiphene citrate or gonadotrophin treatment) since the last ART cycle and/or clomiphene citrate or gonadotrophin treatment prior to starting GnRH agonist therapy,
11. Ovarian enlargement or cyst of unknown aetiology,
12. Ovarian, uterine or mammary cancer,
13. A clinically significant systemic disease,
14. Known infection with Human Immunodeficiency Virus (HIV), Hepatitis B or C virus in the trial subject or her male partner,
15. Abnormal gynaecological bleeding of undetermined origin,
16. Known allergy or hypersensitivity to human gonadotrophin preparations,
17. Any active substance abuse or history of drug, medication or alcohol abuse in the past 5 years prior to the screening visit,
18. Entered previously into this trial or simultaneous participation in another clinical trial.