Exam 3 - Establishing an Accurate Gestational Age and EDD

Question 1

EDDs are calculated based on ___ since LMP

Answer 1

Calculated based on 9 months/40 weeks/280 days since LMP or last normal menstrual period (LNMP)

• LNMP is first day - some may mistakenly provide last day or last expected menstrual period

Question 2

Ways in which reported LMP can be inaccurate

Answer 2

Implantation spotting when blastocyst invades the endometrial lining can occur close to the time the woman is expecting a menstrual period

• DONT count spotting

Question 3

What is Naegel’s rule?

Answer 3

LMP + 7 days - 3 months = EDD

• Based on assumption of 28 day menstrual cycle

Question 4

By how many days are EDDs using Naegel’s rule usually accurate?

Answer 4

10 day margin of error (+/- 5 days) before or after dates

Question 5

What effect does using OCP have on EDD?

Answer 5

5x more likely to deliver post date

• Margin of error correct if no hormonal contraceptive use in past 3 months

Question 6

How is home ovulation monitoring done?

Answer 6

Ovulation occurs every 14 days before next period, so take the date of LMP and count backwards 14 days to determine the day when the woman last ovulated

Question 7

How are pregnancy wheels used to predict EDD?

Answer 7

There is a 5 day variation with pregnancy wheels

• Can be more accurate than other methods since it takes into account calendar variations (e.g. leap years)

Question 8

When should the provider use first trimester ultrasounds to confirm due dates?

Answer 8

Probable signs based on physical exam or when uncertain of LMP d/t lactation, amenorrhea, irregularities of hormonal contraceptive use

Question 9

What method is the most accurate to determine EDD?

Answer 9

First trimester ultrasound

• Accurate within 5 days

Question 10

How do second trimester ultrasounds create EDDs?

Answer 10

• Combined measurement of head circumference by parietal diameter
• Femur length
• Abdominal circumference

Question 11

How accurate are second trimester ultrasounds?

Answer 11

Accurate within 7 days

Question 12

When where is a discrepancy between EDD by LNMP and ultrasound, what should the provider do during first trimester and second trimester?

Answer 12

• First trimester - go with ultrasound date if the ultrasound is >7 days different than the date you came up with by calculation
• Second trimester - use second trimester ultrasound results if the ultrasound is >10 days different than calculation

Question 13

What is a less reliable method of dating pregnancy?

• Measurement of fundal height

Answer 13

Measurement of fundal height - generally correlates uterine size with gestational age

• Factors that influence fundal height
  
  • Bladder distention
  • Maternal body mass
  • Position of uterus
  • Position of fetus
  • Amniotic fluid volume

Should not be used solely to determine gestational age or fetal growth

Question 14

What is a less reliable method of dating pregnancy?

• Pubis-uterine fundus measurement

Answer 14

Should be taken by the same provider throughout pregnancy

• Can feel fundus as early as 12 weeks
• Used as secondary tool to confirm gestational age based on LMP

Question 15

What is a less reliable method of dating pregnancy?

• Quickening and first documented fetal heart tones

Answer 15

Less reliable but helpful