1st Trimester US
From Radipedia
Fetal Heart Beat
- Should be detected if embryo is >5 mm (CRL) or >6 weeks.
- Lower limit of normal is 90 bpm at 5-6 weeks or 120 bpm by 8 weeks
- Normal during 2nd and 3rd trimester is 120-180 bpm
bHCG
- Double every 48 hrs
- At 2000 gestational sac should be seen
Mean Gestational Sac Diameter (MGSD)
- Can be used to estimate gestational age when fetal pole not present
- Caution "pseudogestational sac" seen with Ectopic pregnancy when don't see yolk sac
Yolk Sac should be visualized
- Transvag when gestational sac (MGSD) equals 8mm
- Transabd when gestational sac (MGSD) equals 20mm
- Yolk sac >6mm - recommend follow-up as most will result in failed pregnancy
Fetal Pole should be visualized
- Transvag when gestational sac (MGSD) equals 16mm
- Transabd when gestational sac (MGSD) equals 25mm
Nonviable Pregnancy
- Nonvisualization of a yolk sac with mean sac diameter of 13 mm
- Nonvisualization of an embryo with mean sac diameter of 18 mm
- Nonvisualization of cardiac activity with embryo of 5 mm in length
Nuchal Translucency
- Measured at 11-14 weeks
- Normal is <3mm
- Associated with aneuploidy (Trisomy 21)
- Nuchal fold measured during 2nd Trimester US
Corpus luteal cysts of pregnancy should resolve by 16 weeks.
Use the term embryo instead of fetus in first trimester.
