  We report on the timing analysis of the 2015 outburst of the intermittent
accreting millisecond X-ray pulsar SAX J1748.9-2021 observed on March 4 by the
X-ray satellite XMM-Newton. By phase-connecting the time of arrivals of the
observed pulses, we derived the best-fit orbital solution for the 2015
outburst. We investigated the energy pulse profile dependence finding that the
pulse fractional amplitude increases with energy while no significant time lags
are detected. Moreover, we investigated the previous outbursts from this
source, finding previously undetected pulsations in some intervals during the
2010 outburst of the source. Comparing the updated set of orbital parameters,
in particular the value of the time of passage from the ascending node, with
the orbital solutions reported from the previous outbursts, we estimated for
the first time the orbital period derivative corresponding with
$\dot{P}_{orb}=(1.1\pm0.3)\times 10^{-10}$ s/s. We note that this value is
significant at 3.5 sigma confidence level, because of significant fluctuations
with respect to the parabolic trend and more observations are needed in order
to confirm the finding. Assuming the reliability of the result, we suggest that
the large value of the orbital-period derivative can be explained as a result
of an highly non-conservative mass transfer driven by emission of gravitational
waves, which implies the ejection of matter from a region close to the inner
Lagrangian point. We also discuss possible alternative explanations.
