The Ultrasound Scan at 11-14 Weeks
The guidelines recommend management of common obstetric conditions at primary and referral level, based on local experience and evidence-based practice. They are not intended as strict protocols nor as standard operating procedures, but rather as a guide to safe effective management of common obstetric problems. The guidelines should be most useful to interns, medical officers and registrars.
Ideally, all pregnant women should have their first scan between 11 & 14 weeks. Early assessment of mother & fetus result in fewer antenatal visits, less invasive procedures, & better detection rate of congenital abnormalities.
The purpose of the first trimester scan is:
To date the pregnancy accurately
Crown-rump length measurement allows accurate calculation of the gestational age
To diagnose major fetal anomalies
In about 1% of pregnancies major abnormalities may be visible at this scan. These abnormalities may be incompatible with life or may be correctable
To calculate the risk for chromosomal abnormalities
All women, of whatever age, have a chance of delivering a baby with chromo-somal abnormalities, e.g. Down’s syndrome. The risk of a chromosomal abnormality is calculated from the maternal age (MA), gestational age, fetal nuchal translucency (NT), & presence or absence of the nasal bone (NB). Assessment of (MA + NT + NB) gives a detection rate of 92%. With the addition of serum PAPP-A (pregnancy-associated placental protein A), free beta-hCG & Placental Growth Factor (PIGF), the detection rate improves to around 97%. Women with increased nuchal translucency or increased overall risk may be offered CVS or amniocentesis after counseling.
To determine the risk for pre-eclampsia
Increased resistance in the uterine arteries detected by Doppler velocimetry suggests an increased risk for development of pre-eclampsia. A reduced PAPP-A & PIGF serum level are good markers for pre-eclampsia prediction
To determine the risk for preterm labour
The measurement of the cervical length in the first trimester enables the calculation of a risk of preterm labour before 34 weeks
To diagnose multiple pregnancy
Approximately 2% of natural conceptions & 10% of assisted conceptions result in multiple pregnancies. The scan helps determine chorionicity in twins
To diagnose early pregnancy failure
Early pregnancy failure (missed abortion) can be identified in about 3% of women.