Fetal development at 15 weeks of pregnancy At 15 weeks of pregnancy, the fetus has been developing for a period of time. At this time, the fetus is about 10 centimeters in length and weighs about 70 grams. During this period, the fetus has been able to show its true personality and has its own preferences and habits. At this time, the fetus\’s hearing organs have completed development and can begin to hear the mother\’s voice and some external sounds.