3wks GA(=7-10days post fertilization) [HCG 5-50 mIU/ml First International Reference Preparation]: Blastocyst implants into the endometrium.
    4wks [5-425]: decidual reaction
    4.5wks: Gestational sac (~2mm) should be detected with intradecidual sign: asymmetric location from echogenic uterine cavity and double sac sign [>2mm thick] (outer: trophoblast induced maternal decidua parietalis reaction, inner: decidua capsularis and basalis).
            If not asymmetrically placed, may represent uterine cyst/fluid -look for beaking. With no vascular ring of fire or thick decidual reaction (lack of trophoblast reaction), likely pseudogestational sac.
    5 wks [20-7340]: Yolk sac visible until 10wks. Should be <5.6mm else suspect impending demise.
             Yolk sac must be seen when MSD is > 8mm on EV, or MSD >20mm on TA or HCG >5800
    5.5wks: Can get double bleb sign  as the amniotic cavity expands with yolk sac present
    6wks [1080-56500]:  cardiac activity now visible  in embryonic disk (range 90-110bpm), must be seen by end of 6th week with CRL >5mm and [HCG>25000].
    8wks: may see normal lucent Rhombencephalon(forms 4th ventricle). Start to see placenta.
    8-12wks: normal midgut herniation (6-9mm) as rotation is underway.
    12 wks: [peak at 290k] amniotic sac continues to expand away from fetus and yolk sac too small to resolve, and must have resolution of normal midgut herniation. Nuchal translucency is present from 10-14 wks, but must be less than 3mm (in-in wall  Callen 41) and be careful to exclude amnion  If > 3mm suggests abnormal karyotype or congenital heart disease, even if nuchal thickening resolves by 14 wks. NB. This is not the same as the measurements for Down's (> 6mm to outer skin fold obtained at 18 wks).
        Can perform Chorionic Villus Sampling from 10-12wks.
         
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