Why does the sample have to be taken between 14 weeks + 0 days, and 17 weeks + 6 days ?
		Does the sample have to be centrifuged and decanted rapidly ?
		What to do in the case of twins ?
		Can the sample be taken within four days of an ultrasound examination ?

Should long car journeys (of over 50 miles) be avoided in the four days prior to the taking of the sample ? ?

The legislation stipulates that the sample must be taken between the fifteenth and the eighteenth week after the last period (i.e. from 14.0 to 17.6). To help you decide when the sample should be taken, we have produced sampling schedules which can be sent to you.
Interpretable results can be obtained up to week 20; in other European countries, screening between week sixteen and week twenty is allowed.
However, the sample should be taken in the stipulated time frame; otherwise, if the patient proves to fall into a high risk group and fetal karyotyping needs to be done, this procedure might not be reimbursed.
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Does the sample have to be centrifuged and decanted rapidly ?

YES. To avoid dissociation of the hCG into free sub-units, the sample must be centrifuged and decanted rapidly.
It is also vital that the sample is taken into a dry tube.

What to do in the case of twins ?

Maternal serum markers can be used to give an overall estimation of the risk associated with a pregnancy without giving any information on whether one or both of the fetuses might be affected. Moreover, no reference figures exist for gemellar pregnancies in which one or both of the fetuses are trisomic.
Therefore, it is necessary to inform the mother-to-be that this risk is really a pseudo-risk which is much less evaluated than for monofetal pregnancies. The only reliable way of assessing an individual risk factor for each of the two fetuses is the nuchal clarity index at week twelve.
  

Can the sample be taken within four days of an ultrasound examination ?t ?

At the beginning of the use of screening procedures based on just hCG levels, it was common to hear that an ultrasound examination can cause increased hCG levels lasting several days.
In fact, although this idea was often voiced, no quantitative investigations were ever undertaken although it could be true if the placenta is far forward.
In France, several different markers have to be assayed which should mitigate the effects of this phenomenon, if it is real.
 

Should long car journeys (of over 50 miles) be avoided in the four days prior to the taking of the sample ?gemel

This idea that riding in a car can affect hCG levels has never been quantitatively demonstrated and seems somewhat out of date.
Currently, all we know is that high hCG levels during the fourth month of pregnancy have some predictive value when it comes to certain pathological situations.