The penis spirals towards the vagina and into the cervix where it screws into the folds to become locked. This is vital because to establish an optimum pregnancy, sperm must be placed directly into the womb.

Sperm is transported from the cervix to the top of the womb by contractions of the muscles and it takes up to 2 hours for them to arrive at the bottom of the fallopian tubes, the utero-tubal junction. Once the junction is filled with sperm few more enter before ovulation. It is important to appreciate this because at the first mating the quality of the semen and timing probably decide the quality and success of fertilisation and subsequent embryo survival. (Fig.5-36).