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First and early scans
Before 11 weeks gestation
YOUTUBE LECTURE: # Section 1: Early Scans < 10 weeks.
The first time a scan is done it is important and valuable to examine both the abdomen and pelvis.
A. Start by touring the abdomen and pelvis swiftly with the scan, you will be amazed at some findings that would just go un-noticed (as renal cysts, hydronephrosis, gall bladder stones, the cropus luteum cyst etc.) do not say that you can not diagnose such problems since you are an obstetrician, with time your eyes will be familiar with the normal views and you later spot abnormalities and refer her to radiology department. Another good thing is the patients reaction to what you are doing, she will definitely appreciate that you are the first obstetrician to "have a look around” instead of just taking CRL and fetal heart.
B. Move to the pelvis, before looking into the uterus, search the adnexa for masses and/or cysts then have a good look at the uterus itself as it is easier to visualize fibroids or uterine anomalies as septate or bicornuate uterus in early scans.
ovarian cyst with pregnancy - Bicornuate gravid uterus
C. The conceptus itself is then examined;
- The first thing to spot is at 5 weeks on-wards is the gestational sac, it should be in the uterus to exclude ectopic pregnancy.
Do not forget to record the number of gestational sacs...!
- Detecting fetal pole starting from around 6 weeks gestation: look for a white dot in the sac this is the fetal pole.
- Embryonic morphology is not the aim at this gestation although expert level III scan can accurately examine embryonic morphology.
Absent fetal pole may be due to an early scan when still a sac is visualized but not a fetal pole or may be blighted ovum (anembryonic sac) which is diagnosed if MSD (mean sac diameter) is > 2cm without fetal pole or if re-scan after 2 weeks still can not detect fetal pole.
- Extraembryonic structures from outer to inner are the chorionic sac then inside it the amniotic sac. The yolk sac is seen as a small ring that lies within the chorionic cavity and outside the amniotic sac (after 7 weeks). The yolk sac is connected to the fetus through the vitelline duct.
MSD; Mean sac diameter; The gestational sac is measured in three dimensions, and the average, the Mean Sac Diameter (MSD) used for estimating gestational age. It is useful between 5 and 8 menstrual weeks with accuracy of +/- 3 days .
Crown-rump length: Measuring fetal crown-rump length: crown-rump length (CRL) is the measurement from the top of the head (crown) to the bottom of the buttocks (rump). It is measured as the largest dimension of embryo, excluding the yolk sac and extremities. Gestational age estimation is most accurate by CRL measurement, it carries less than 5 days falacy while 2nd trimester BPD only carries 7 days falacy, third trimester scan is not accurate to estimate gestational age.
It has been reported that MSD (mean sac diameter) should exceed CRL by at least 5 mm (i.e. MSD - CRL = > 5mm) as a parameter of healthy pregnancy, patients with MSD - CRL = < 5mm are very prone to first trimester abortion despite a normal heart rate. Chromosomal anomalies particularly Trisomy 18 and triploidy are markedly associated with growth restriction i.e. decreased crown rump length.
The practical sign of fetal well-being at this early gestation is:
- Detecting fetal pulsations, starting from 6 to 8 weeks gestation, presence of arrhythmias or bradycardia is an ominous sign.
- Size of the yolk sac. Small or Large yolk sac (<3mm or > 7mm at 6 to 9 weeks) is as well a bad sign
- Size of the gestational sac compared to CRL (see above)