Is there any relationship between maternal weight gain during pregnancy and gestation period/duration for singletons? for twins?

Not consistantly: Too little weight gain or excessive weight gain could be associated with indications for early delivery (iugr, pre-eclampsia, gestational diabetes, ect). There is no specific concrete direct relationship between pregnancy duration and maternal weight gain.

PREGNANCY: Weight gain is more for twins , gestation age should be term and this is why OB checkups are crucial to attain the right weight gain and duration of pregnancy.It depends on outcome of pregnancy.