Final answer:
Ultrasound is the most accurate method to estimate the Expected Date of Delivery (EDD) if the Last Menstrual Period (LMP) is unknown, as it allows healthcare providers to measure the fetus and determine gestational age.
Step-by-step explanation:
The most accurate method for estimating the Expected Date of Delivery (EDD) when a patient is unsure of their Last Menstrual Period (LMP) is through ultrasound. Using ultrasound, healthcare providers can measure the fetus to determine its gestational age and estimated due date. An ultrasound can accurately measure the size of the fetus and estimate its age. Fetal kick counts and maternal weight gain can provide some information about the pregnancy progress but are not as accurate as ultrasound. Conception date calculation may also be used if the patient has knowledge of when conception occurred.
Other methods like fetal kick counts and maternal weight gain do not provide precise information for dating a pregnancy and therefore, are not reliable for determining EDD. Calculations based on the conception date can be helpful if the actual date of conception is known; however, this date is often unclear or difficult to ascertain with certainty.