Under condition of 經期準 and standard period of 28 days, doctor use your LMP(Last Menstrual Period) to caculate EDD(Expected Date of Delivery). Formula = :Month of LMP +9/-3, Date of LMP +7.
If 經期唔準, or peroid is longer than 28 days or shorter, doctor will use this formula as reference, and use ultrasound to measure baby size.