Fertility treatment is a juggling act. Doctors need to find a balance between success and patient discomfort; between complications and cost. When it comes to choosing the right IVF treatment for you, it’s important to know how your overall experience might be improved. One of the best ways is through Mild ovarian stimulation.

What is MOS?

Mild ovarian stimulation (MOS) is an IVF protocol which is much more convenient and tolerable for patients but more difficult and less convenient for doctors and IVF units. In MOS, lower doses of FSH, or follicle stimulating hormone, injections are given for fewer days, the aim is to limit the number of eggs collected to around four.

Why lower doses of FSH?

FSH is the hormone that regulates the female body’s reproductive system and is given to women going through IVF to produce more than one egg to grow to maturity. Usually, high levels of FSH are given by most IVF doctors, however a new study[1] has shown that the higher the dose of FSH used, the lower the associated chance of a baby. The overall live birth rates reported show twice the rate for the low stimulation dose compared with the highest stimulation dose! And not only are there higher baby rates with lower FSH doses, there are less side effects.

FSH is commonly given during IVF cycles at doses that allow retrieval of multiple eggs, with the goal of increasing the chances of live birth above what would have been possible with retrieval of just one egg. Although it is generally agreed that there is benefit to the retrieval of multiple eggs, it is now recognised that the abnormal hormonal environment generated by ovarian stimulation may have negative effects on both the endometrium (womb lining) as well as egg maturation.

The usual protocols that are currently applied in most IVF clinics[2] have a target of generating between 8 and 15 oocytes. These protocols are complex, time consuming and expensive and may give rise to considerable patient discomfort and chances for complications. Now, there is evidence that collecting lots of eggs can negatively affect embryo quality. This may be due to interference with natural selection of the best-quality eggs or other repercussions of ovarian stimulation on eggs, womb lining, or embryo quality. Given the potential for adverse consequences of ovarian stimulation on the endometrium, egg, or embryo, there is increasing interest in mild ovarian stimulation for IVF with the goal of retrieving a limited number of eggs.

Advantages of MOS

Mild ovarian stimulation has a number of advantages. These include:

  1. Similar live birth rates per treatment

    Studies have shown[3] that the term live birth over a 1-year treatment period of MOS IVF is similar compared with conventional IVF

  2. Reduced complexity, patient discomfort and risk

    Mild stimulation uses lower doses and fewer days of FSH. It is a less complex process for the patient and reduces patient distress and complications such as OHSS (ovarian hyperstimulation syndrome) which can be very distressing and require admission to hospital. It is also more convenient as it also reduces the need for frequent visits to the clinic for the intense monitoring of ovarian response. MOS protocols have been shown to decrease drop-out rates and to allow a higher acceptance of repetitive IVF cycles. And this increases a woman’s chance of a baby!

  3. Reduced cost

    Improved overall health economics of mild IVF treatment has been reported. When mild stimulation is combined with a single embryo transfer policy, costs associated with pregnancy complications were dramatically decreased

  4. Beneficial effect on egg/embryo quality

    The essential aim of mild stimulation is to remain as close as possible to normal ovarian physiology, allowing for only few follicles to continue their development. Studies suggest that this approach is beneficial for both egg/embryo quality and endometrial receptivity

MOS has been developed in the pursuit of fertility treatments that maximise treatment efficacy and live birth outcome. MOS is much harder to control for the doctors, and is more likely to be associated with ‘messy’ cycles because it relies so much on the woman’s underlying ovarian function. This means that the doctor must understand how the ovary works really well, and also then must be able to explain why these things happen to you. Frankly, if your doctor can’t explain how your ovaries are working, you should ask yourself why you are letting them treat you.

The most important thing is to stay focused on the big picture. IVF treatment is about maximising your chance of having a baby. It’s not about how many eggs or embryos you get. It’s about the quality.

Before undertaking IVF, be sure to do your research and speak to a number of doctors in detail about their approach; and remember are that higher doses of FSH decrease your chances of having a baby, and that more eggs do not equal more babies. All women are different, so IVF treatment cannot be a ‘one size fits all’ approach.


[1] Fertility & Sterility, November 2015. Gonadotropin dose is negatively correlated with live birth rate: analysis of more than 650,000 assisted reproductive technology cycles

[2] Human Reproduction, November, 2010. Mild Ovarian Stimulation for IVF: 10 Years Later.

[3] Human Reproduction, November, 2010. Mild Ovarian Stimulation for IVF: 10 Years Later.