Final answer:
Fetal movements are most frequent and strong during the third trimester, specifically from week 29 to birth. Quickening, or initial movements, can be noticed as early as weeks 16-20 but become stronger and more noticeable as the fetus continues to grow and space becomes restricted.
Step-by-step explanation:
The movement of a fetus is likely to be most frequent and strong during the third trimester, which occurs during weeks 29 through birth (at about 40 weeks). During this period, the uterus expands rapidly, resulting in more rapid weight gain for the pregnant person and stronger and more frequent fetal movements that may become disruptive. As the fetus grows larger, it has less space to move, which makes its movements more noticeable to the pregnant person.
It's worth noting that fetal movements can be felt as early as weeks 16-20 in what is known as 'quickening.' The fetus grows to about 30 cm (12 inches) during the second trimester and becomes more active, leading to the pregnant person feeling the first movements. However, these movements grow increasingly strong and frequent as the fetus matures towards birth.