1. Indications and Preparations

B-hCG Discriminatory Zone

It is generally accepted that an intrauterine pregnancy should be seen on transvaginal ultrasound with a B-hCG level of 1000-2000 mIU/mL, and on transabdominal ultrasound with a level of 3000-5000 mIU/mL; this is called the discriminatory zone (Bree et al). If an intrauterine pregnancy (IUP) isn’t seen at this time, you should be suspicious of an ectopic pregnancy or non-viable intrauterine pregnancy.

However, there has been some debate on the utility of the discriminatory zone, with evidence showing the chance of a viable IUP developing later on despite no IUP detected below the discriminatory zones.

1. Indications and Preparations

Pre-Exam Preparation

Determining Estimated Due Date Based on LMP

It is important to estimate the gestational age prior to Obstetric/OB ultrasound assessment to determine if the findings are normal. Naegele’s Rule is a simple way to calculate the due date based on the last menstrual period (LMP) and estimate gestational age prior to the Obstetric/OB ultrasound assessment.

Due Date = Date of first day of last menstrual period (LMP) + 9 Calendar months + 7 days

For example, if the LMP was 7/16/20, add 9 months (4/16/21), then add 7 days = 4/23/21

Gestational age is estimated in weeks from the first day of the LMP. The gestational age as well as the estimated due date can be refined using ultrasound findings which we’ll discuss below.

1. Indications and Preparations

Obstetric/OB Ultrasound Indications

  • Vaginal bleeding in pregnancy
  • Abdominal/Pelvic pain in pregnancy
  • Gestational dating
  • Positive pregnancy test with increased B-hCG
  • Assessing for fetal viability
  • Maternal trauma