WEEK 5 OF GESTATION

• Embryonic pole can be distinguished at 5 weeks and 3-4 days.
• Embryo ± 2mm. Growth = 1mm/day.

Normal video at 5 weeks of gestation - CRL = 4mm

• Heart.
• Cardiac activity begins at 5 weeks 2 days, but may not be appreciated until embryo = 5mm in size, but should always be seen when embryo > 5mm (6 weeks 4 days).
• Cardiac activity can usually be detected endovaginally on day 27-29 or as soon as the embryonic pole is visualized (1).
• The heart rate is only 70-90 beats / minute (2).
• Primitive and two chambered, with blood flow from the venous to the arterial side in a sequential manner.
• Ventricular chamber enlargement occurs at about 40 days from LMP.
• Gestational sac can be identified as early as 4 weeks 3 days and can be routinely detected endovaginally after 5 weeks.

• Double Bleb.
• Double sac structure, which represents the developing yolk sac and extraembryonic coelom.
• Present for only 2-3 days.

	4^+5 weeks · Gestational sac · Yolk sac not visible
	5^+1 weeks · Gestational sac · Yolk sac visualized.

• Configuration of the Embryo.
• The embryo is initially visualized as an echogenic area of thickening on the yolk sac.

• Straight until it reaches 3-4mm in length (on ultrasound the embryo is a straight echogenic line adjacent to the yolk sac and close to the vitelline duct). Occasionally it cannot be seen, but the prominent pulsations of the embryonic heart reveal its presence.

• Flexion into a C-shape as the embryo grows. (greatest length of embryonic pole = neck rump length as head is flexed).
• CRL can be measured at ± 9 weeks GA when fetus = 18-22mm.

REFERENCES

1. Schats R, Jansen AM, Wladimiroff JW. Embryonic heart activity: appearance and development in early human pregnancy. Br J Obstet Gynaecol 1990;97:984-994.
2. Levi CS, Lyons EA, Zheng XH et.al. Endovaginal ultrasound: demonstration of cardiac activity in embryos less than 5mm in crown-rump length. Radiology 1990;176:71-74.