Usually the follicle develops to 18mm before it is relatively mature and able to ovulate and fertilize normally, at 16mm it is usually not able to fertilize normally.
Under normal circumstances, the follicle grows at a rate of about 2mm per day. When the follicle is 16mm, it will not ovulate, and it is difficult for it to combine with sperm to form a fertilized egg, so it cannot conceive, but it may grow in the next day or two, and when it matures, it can ovulate and conceive normally.
It is recommended that women who need to prepare for pregnancy should strengthen nutrition, eat a balanced diet, dynamically observe their own body changes, and increase the frequency of coitus to increase the probability of conception. If you are not planning to get pregnant, you should take active contraceptive measures when having sex during this period to prevent unwanted pregnancy.