Worried about a shrinking population, Japan retailers have discovered new ways for women to detect their most fertile moments including a ringing phone.

A new phone is available that calculates ovulation times according to dates of her menstrual period and alerts her three days beforehand. The phones have been made available through Japan's NTT DoCoMo.

The phone was developed after the country's fertility rate reached an all-time low. The rate represents the average number of children a woman bears in her lifetime.

Phone designer Momoko Ikuta also installed other features that appeal to women. This includes memory for cooking recipes and even a ringer that goes off when a woman wants to ward off unwanted attention.