Rationale: Due to accelerated germ cell loss, infertility is a major problem in girls with Turner syndrome (TS). Therefore, cryopreservation of ovarian tissue or oocytes before exhaustion of the ovarian reserve may preserve fertility in patients with TS. However, in the majority of females with TS , the ovarian reserve is exhausted before the age of menarche. Early markers indicating and predicting the ovarian reserve are necessary. During mid-childhood the hypothalamic-pituitary-gonadal (HPG) axis is quiescent and gonadotropins are usually unmeasurable. Nonetheless, this axis is active during infancy. Therefore, gonadotropins are measurable with peak values at 3 months of age and with lower (but still measurable) values at 9 months of age, in a period called the minipuberty. The aim of this study is to find markers of ovarian capacity, during the minipuberty, in order to predict ovarian reserve in the future.

Objective: The hormonal range of LH, FSH, AMH, inhibin B, testosterone and estradiol in girls with TS during the minipuberty and the relation of the hormone serum levels with the karyotype.

Study design: A prospective, cohort study with a duration of 3 years. Study population: Girls with a pre- or perinatal diagnosis TS who are born in a medical centre in the Netherlands during the duration of the study

Main study parameters/endpoints: Serum levels of FSH, LH, AMH, inhibin B, testosterone and estradiol at the age of 3 and 9 months.

Nature and extent of the burden and risks associated with participation, benefit and group relatedness:

The subjects will have twice an extra venapunction for collection of 3.5mL blood during their infancy, which is not stated in the guidelines for TS. There is very little risk for adverse events associated with this blood sample collection, however it is an extra procedure. The outcome parameters will not be helpful for individual study participants, however they are likely to help clinicians and researchers in understanding how the ovarian function operates develops in girls with TS. Furthermore, these markers could be used to estimate the ovarian reserve and the urgency of fertility preservation in young females with TS. This information could help clinicians, patients and their parents in decision making.

Inclusion Criteria:

In order to be eligible to participate in the TS group of this study, a subject must meet all of the following criteria:

• A diagnosis of TS before the age of three months;
• Girls with a diagnosis of classic TS or other variants (i.e. 45,X, 45,X/46XiXq, 45,X/46,XY, 45,X/46,XX, 45,X/47,XXX, 45,X/46,X,r(X), 46,XiXq, other);
• Whose parents have agreed to participate in the study through a signed written informed consent form.

In order to be eligible to participate in the control group of this study, a subject must meet all of the following criteria:

• No diagnosis of TS or any other diagnosis that might affect the HPG axis;
• Girls that will have a blood collection within their usual care at 3 months and at 9 months of age.
• Whose parents have agreed to participate in the study through a signed informed consent form.

Exclusion Criteria:

A potential subject who meets any of the following criteria will be excluded from participation in this study:

• Any other diagnosis besides TS that might affect the HPG axis;
• Ovarian surgery in the medical history;
• Critical illness;
• The use of medication affecting the HPG axis (e.g. estrogen suppletion therapy)

We include girls with the diagnosis TS before the age of 3 months based on full or partial absence of one X-chromosome or a mosaic pattern. We will recruit the girls with TS from all medical centres in the Netherlands. The control group consists of girls who have blood collected at 3 months and at 9 months of age for another indication. Participants in the control group will be recruited in the Radboudumc in Nijmegen.