Which of the following statements about Rhythm method is false?

Anencephaly is not a repoed complication of calender method. The calender method or rhythm method: By this method, couple is advised avoidance of sexual intercourse around ovulation. In a 28-day cycle, ovulation normally occurs on the 14th day of the cycle, but may occur anytime between the 12th and 16th day. Spermatozoa deposited in the female genital tract may survive for 24 hrs. The ovum itself may live for 12-24 hrs. Hence intercourse between the 11th and 17th day may result in a pregnancy. The safe period is, therefore calculated from the first day of the menstrual period untill the 10th day of cycle and from the 18th and 28th day. An alternative method is to calculate the risk period, which is from 3 days before ovulation to 3 days after ovulation. In a 35-day menstrual cycle, therefore, ovulation will occur on the 21st day (that is 14 days before the nexe period) so that the risk period is from day 18 to day 24 Ref: Shaw&;s Textbook of Gynaecology 17th edition Pgno: 253