First-Trimester Ultrasound Dating and Nuchal Translucency Scan

Introduction to First-Trimester Ultrasound Dating

First-trimester ultrasound dating is highly reliable and should take precedence over menstrual dating when discrepancies exist, due to its accuracy, as supported by the American College of Obstetricians and Gynecologists (ACOG) 1
The embryo transitions to being called a fetus around 10 weeks, and crown-rump length (CRL) measurements are used for dating at this stage 2

Incorrect menstrual dating is the most common explanation for a fetus larger than expected based on menstrual dating, as conception may have occurred earlier than calculated from the last menstrual period, with a strength of evidence level I 1
Multiple gestation, such as twins, can sometimes cause measurement discrepancies, although this was not indicated in the provided case, as noted by the Society for Maternal-Fetal Medicine (SMFM) 3
Discrepancies between uterine size and gestational age may indicate incorrect dating, multiple gestation, uterine fibroids, gestational trophoblastic disease, growth restriction, or macrosomia, highlighting the importance of careful interpretation of ultrasound findings 4

CRL measurements at 11-13 weeks provide an accurate estimate of gestational age within 7 days, and should be used to adjust the dating, as recommended by the National Institute for Health and Care Excellence (NICE) 1
The new gestational age should be expressed as 13 weeks + 5 days rather than rounding to completed weeks, for precision, with a moderate strength of evidence 1

NT screening performance is optimal at 11 completed weeks and decreases by 13 completed weeks, emphasizing the importance of timely screening, as supported by the Fetal Medicine Foundation (FMF) 1
The NT measurement remains valid if performed within the 11-13+6 weeks window, allowing for some flexibility in screening timing, with a high strength of evidence 5
The measurement should be interpreted using the appropriate reference range for the new gestational age to ensure accuracy, as noted by the International Society for Prenatal Diagnosis (ISPD) 1

If the NT measurement is ≥3mm or ≥99th percentile, genetic counseling and diagnostic testing should be offered promptly to address potential concerns, as recommended by the American College of Medical Genetics and Genomics (ACMG) 5
A detailed anatomy scan at 18-22 weeks should still be performed as scheduled, regardless of NT measurement outcomes, for comprehensive assessment, with a moderate strength of evidence 3

When CRL measurements differ from menstrual dating by more than 7 days in the first trimester, the due date should be adjusted based on the ultrasound to reflect the most accurate gestational age, as supported by ACOG 1
Even with the dating adjustment, NT screening is still valid if performed within the 11-13+6 weeks window, ensuring that screening opportunities are not missed, with a high strength of evidence 5
Without other concerning findings, a larger-than-expected CRL is most likely due to dating error rather than pathology, emphasizing the need for careful interpretation of ultrasound findings, as noted by the SMFM 1

The uterus at 12 weeks is approximately the size of a grapefruit, begins to protrude from the pelvis, and is softer and more globular in consistency compared to the non-pregnant state, with Chadwick's sign present, as described by Williams Obstetrics 6
The uterus is palpable above the pubic symphysis at 12 weeks, which is an important clinical milestone that confirms appropriate pregnancy progression, as noted by the American Academy of Family Physicians (AAFP) 4
The cervix becomes softer (Goodell's sign) compared to the firm non-pregnant cervix, which is a significant physical examination finding at this stage, as described by the Journal of Family Medicine 6