What Each Pregnancy Ultrasound Indicates
First Trimester Ultrasound (Before 14 Weeks)
The primary purpose of first trimester ultrasound is to confirm intrauterine pregnancy location, establish accurate gestational dating, and exclude ectopic pregnancy—which occurs in up to 13% of symptomatic emergency department patients. 1, 2
Key Indications and Findings:
Confirm intrauterine pregnancy and exclude ectopic pregnancy: This is the most critical early function, as 36% of ectopic pregnancies present with β-hCG <1,000 mIU/mL, and ultrasound can detect 86-92% of ectopic pregnancies even at low β-hCG levels 2, 3
Establish accurate gestational dating: Early scanning provides exact dating with minimal error, which is essential for managing the entire pregnancy 4, 5
Determine number of fetuses and chorionicity: Multiple pregnancies must be identified early, as chorionicity can only be correctly established before 14 weeks of gestation—this is crucial because monochorionic twins have a 10% mortality rate and require intensive surveillance 1, 4
Visualize early pregnancy structures by gestational age:
Nuchal translucency measurement (11-14 weeks): Screens for Down syndrome, trisomy-18, trisomy-13, Turner syndrome, and other aneuploidies with approximately 80% detection rate 1, 4
Detect major structural abnormalities: Early diagnosis of anencephaly, megacystis, polycystic kidney, and omphalocele is possible, allowing for different clinical management 4, 6
Critical Pitfall to Avoid:
Never defer ultrasound based solely on low β-hCG levels—algorithms that wait for a "discriminatory threshold" result in diagnostic delays averaging 5.2 days, and some patients show evidence of rupture at eventual diagnosis. 2, 3
Anatomy Scan (18-22 Weeks)
The American College of Radiology recommends at least one ultrasound between 18-20 weeks for all pregnant women to evaluate fetal structure and development—this is the standard screening examination. 7, 8
Key Indications:
Comprehensive fetal anatomical survey: Systematic evaluation of fetal organs, limbs, spine, heart, brain, face, and abdominal structures to detect major malformations 8, 9
Placental location assessment: Identifies placenta previa, low-lying placenta, or vasa previa, which directly impacts delivery planning 1
Amniotic fluid volume assessment: Evaluates for oligohydramnios or polyhydramnios 1
Fetal biometry and growth assessment: Establishes baseline measurements for monitoring growth throughout pregnancy 8, 9
Special Considerations:
For maternal obesity (BMI ≥35 kg/m²): Delay anatomy scan to 20-22 weeks due to suboptimal visualization, with repeat follow-up in 2-4 weeks if incomplete 7
Transvaginal ultrasound may supplement transabdominal scans when fetal structures are difficult to visualize, particularly in obese patients 7
Third Trimester Ultrasound (After 28 Weeks)
Third trimester ultrasounds are not routinely performed in low-risk pregnancies, as routine screening has not shown evidence of improved outcomes—however, they are indicated for specific clinical concerns or high-risk conditions. 7
Specific Indications:
Fetal growth assessment: Evaluates for intrauterine growth restriction or macrosomia, particularly important in maternal obesity (BMI ≥35 kg/m²) where clinical assessment is limited 7
Follow-up of second trimester findings:
- Isolated echogenic bowel: Third-trimester reassessment and growth evaluation 7
- Isolated single umbilical artery: Growth evaluation with consideration of weekly antenatal surveillance beginning at 36 weeks 7
- Isolated urinary tract dilation: Follow-up at 32 weeks 7
- Isolated shortened humerus or femur: Reassessment and growth evaluation 7
Placental location confirmation: Re-evaluate placenta previa or low-lying placenta identified earlier 1
Amniotic fluid volume: Assess for oligohydramnios or polyhydramnios 1
Umbilical artery Doppler velocimetry: Evaluates placental function and fetal well-being, particularly in growth restriction or preeclampsia 1, 7
Multiple Gestation Surveillance
Multiple pregnancies require substantially more frequent monitoring than singleton pregnancies, with the specific schedule determined by chorionicity. 1, 7
Dichorionic Twins:
- First trimester dating scan 1
- Nuchal translucency scan at 11-14 weeks 1
- Anatomy scan at 18-22 weeks 1, 7
- Serial growth scans every 3-4 weeks starting from anatomy scan 7
Monochorionic Twins (Higher Risk):
Monochorionic diamniotic pregnancies have a 10% mortality rate, primarily from twin-twin transfusion syndrome (TTTS) and discordant fetal anomalies—intensive surveillance is mandatory. 1
- Begin surveillance at 16 weeks 7
- Weekly to biweekly monitoring for TTTS: Assess amniotic fluid volumes and bladder visualization 7
- Fetal biometry every 2-3 weeks 7
- Umbilical artery Doppler evaluation incorporated into surveillance 7
- Twin anemia-polycythemia syndrome (TAPS) occurs spontaneously in approximately 5% of monochorionic diamniotic twins 1
Monochorionic Monoamniotic Twins (Highest Risk):
- Approximately 1% of monozygotic twin pregnancies 1
- Survival rates >90% with early diagnosis, serial sonograms, and antenatal surveillance 1
- Undergo preterm cesarean delivery to avoid cord compromise and fetal demise 1
Vaginal Bleeding Evaluation (Second and Third Trimester)
Ultrasound is the backbone of evaluating second and third trimester vaginal bleeding, with transvaginal and transabdominal approaches both usually appropriate. 1
Key Indications:
Painless vaginal bleeding: US pregnant uterus transabdominal, US pregnant uterus transvaginal, and US duplex Doppler velocimetry are usually appropriate 1
Painful vaginal bleeding: Same ultrasound modalities are usually appropriate 1
Suspected or known placenta previa, low-lying placenta, or vasa previa: Transabdominal and transvaginal ultrasound with Doppler are usually appropriate, but transperineal cervical ultrasound is usually not appropriate 1
Important Caveat:
The outcome of pregnancies is unequivocally related to the specific etiology of vaginal bleeding—knowledge of imaging results directly informs patient management to optimize outcomes for mother and fetus. 1
Advanced Imaging Considerations
Fetal MRI:
- Increasingly used for pregnancies complicated by congenital anomalies or complications related to monochorionic gestations 1
- Helpful for assessing intracranial injury following spontaneous single fetal demise in monochorionic twins or after laser ablation of intertwin vascular connections 1
- Gadolinium contrast is relatively contraindicated in pregnancy with no established indications for fetal evaluation 1