The technique is even more streamlined than one announced a few weeks ago which requires a father's saliva and a mother's blood.
This latest method is reported in the journal Nature. It can be done without knowing who the father is.
The father-free method promises to make fetal sequencing possible in every pregnancy.
Determining a fetus's genome might give women reasons to end a pregnancy. But researchers say it would also let doctors identify conditions that can be treated before birth or immediately after.
