Question

What is the rule of thumb for the relationship between using US and LMP to date a pregnancy?

a) If US dating differs by more than 7 days from LMP dating, use LMP
b) If US dating differs by more than 10 days from LMP dating, use LMP
c) If US dating differs by more than 14 days from LMP dating, use US
d) If US dating differs by more than 21 days from LMP dating, use US

Answer 1

The correct rule of thumb for dating a pregnancy is to use ultrasound dating if it differs by more than 14 days from LMP dating.

Step-by-step explanation:

The rule of thumb for the relationship between using ultrasound (US) and the last menstrual period (LMP) to date a pregnancy varies. The option that is generally accepted as the rule of thumb is:

• If US dating differs by more than 7 days from LMP dating, in the first trimester, use the US date.
• If US dating differs by more than 10 days from LMP dating, in the second trimester, use the US date.
• If US dating differs by more than 14 days from LMP dating, in the third trimester, use the US date.

So, the correct answer is 'c) If US dating differs by more than 14 days from LMP dating, use US'. The goal is to provide the most accurate dating for the pregnancy, which affects management and expected date of delivery.