THE NORMAL GESTATIONAL SAC

• First intrauterine structure seen in the pregnant uterus.
• Visualized
• 4.0 weeks + 1-2 days transvaginally.
• 5 weeks transabdominally.

ULTRASOUND

• Embryonic Vesicle (Primary yolk sac)
• Round or oval.
• 1-2mm (4 weeks 2 days gestational age), and grows to 2-4mm at 32-35 days and is 5mm at 36-37 days (menstrual age).
• Ring of decidualized endometrium.

	Small gestational sac prior to visualization of the yolk sac or embryo. Note the marked vascularity on power Doppler at the site of implantation.

TRUE GESTATIONAL SAC

• Round or oval, but may be distorted by:
• Full bladder.
• Physiological uterine contractions
• Adjacent mass (leiomyoma or hemorrhage).

Video clip of an elongated gestational sac in a non-viable pregnancy.

• Double decidual sign
• Fundal or mid-portion of the uterus.
• May occasionally implant low down in the uterine cavity.

Low implantation of gestational sac in lower uterine segment
	Case 1 (above) – 5 weeks of gestation Case 2 (left) – 8 weeks of gestation

• The gestational sac implants and is surrounded by a decidual reaction. The echogenic line of the uterine cavity bypasses the gestational sac –

“Intradecidual sign”. This differentiates a low lying pregnancy or cervical ectopic pregnancy from an abortion in progress.

This sign is only reliable prior to 12 weeks GA as the inferior portion of the gestational sac reaches the uterine corpus by 12 weeks GA.

Positive intradecidual sign It is a sign of a true intrauterine pregnancy. It differentiates a true gestational sac from a pseudosac (associated with ectopic pregnancy). It differentiates a low lying pregnancy or cervical pregnancy from an abortion in progress. An intact gestational sac can be seen in cervical pregnancy, low lying pregnancy and passing abortion.

Negative intradecidual sign Conceptus and fluid is in the endometrial cavity and not within the decidua (passing abortion). Other signs associated with a negative intradecidual sign include: 1. Hourglass shape of the uterus due to enlargement of the cervix (80% cervical pregnancy and 25% passing abortion). 2. Enlarged corpus only in passing abortion (25%) but not in cervical pregnancy (in passing abortion the pregnancy was initially in the uterine corpus whereas in cervical pregnancy, the pregnancy was in the cervix from the beginning. 3. External os is more frequently closed in passing abortion (80%) than in cervical pregnancy (60%). 4. Internal os is more frequently open in passing abortion (38%) than in cervical pregnancy (20%).

• Growth - 1-2 mm/day.
• Developing embryonic disc - not yet seen.
• Developing amnion - not yet seen.
• Endometrial cysts or vessels may be seen in the ring of decidualized tissue.
• Lacunar intervillous space of the developing placenta demonstrate low velocity flow on doppler.
• Discriminatory Level = level of bhCG at which an intrauterine gestational sac and contents must be seen to call the pregnancy normal.
• No gestational sac at a specific level
• abnormal pregnancy.
• aborted pregnancy.
• ectopic pregnancy.
• Gestational sac should be seen at: 500-1000mIU/ml (2nd International Standard) Endovaginally. 1800mIU/ml (2nd International Standard) Transabdominally.
• Gestational sac diameter versus gestational age (Table)– Daya et.al. 1991
• Gestational sac diameter versus gestational age (Graph) – Daya et.al. 1991