Normal early pregnancy imaging

NORMAL EARLY PREGNANCY AND

IMAGING

1st trimester : the period from fertilization of the ovum to the start of the 13th post menstrual week.

Imaging approaches

1- Transvaginal ultrasound : is the main stay of early pregnancy as it offers excellent resolution for developmental stages.

2- Transabdominal ultrasound : excellent for rapid confirmation of a live IUP

3- Doppler : the use of Doppler should be avoided in 1st trimester due to concerns regarding heating effect and embryonic damage and some recent studies correlating the abuse of Doppler in 1st trimester and autism. The usual Doppler sign is the “ring of fire” which is a non specific finding as it can be found in ectopic pregnancy and in the corpus luteum as well as normal chorionic tissue

4- M-Mode : is used to document cardiac activity in the embryo

5- MRI : current recommendations are to avoid MRI studies as the its effect on embryo at such early stage is not sufficiently studied

6- CT : as an ionizing radiation it’s contraindicated in 1st trimester except in grave indications as in trauma

7- 3D/4D scanning

Embryologic stages 1- Embryo formation :

-Ovum + sperm = zygote

-Cleavage of zygote = morula

-Central fluid filling of the morula= blastocystic cavity that separates trophoblast from inner cell mass and is divided into

- trophoblast : embryonic part of the placenta

- Inner cell mass ( embryoblast) : Primordium of embryo

- Embryoblast forms a bilaminar embryonic disc - The embryonic disc transforms into a 3D C-shaped

embryo be the beginning of 6 week post menstrual

Timing of some embryonic events Neural tube closes by day 26 (5th-6th

weeks)

Heart is partitioned by the end of 8th week but cardiac activity can be detected prior to that when the embryo reaches a length of 5-6 mm

Limb-buds are seen by 9-10 weeks

Embryo gives the recognizable human form by the end of 10 weeks

Physiologic Bowel herniation : because the bowel grows rapidly than the rest of the embryo it herniates at the base of the cord then it undergoes rotation within the cord then returns to abdominal cavity . Please Note that the liver never herniates normally.

Multiple Pregnancy Multiple pregnancies and types of twinning depends on

the number of zygotes and timing of division and they are :

1- Dizygotic twins : two fertilized ova and they are all dichorionic , diamniotic

2- Monozygotic twins : the amnionicity and chorionicity depends by the time of zygote division

- Before 3rd day post conception : diamniotic, dichorionic - 4th-8th day post conception : monochorionic , diamniotic - Cleavage of the inner cell mass of blastocyst after 8th

day post conception : monochorionic, monoamniotic.- Incomplete cleavage of embryonic disc after 13th day

post conception : conjoined twins

Monoamniotic monochorionic twins

Monochorionic Diamniotic Twins

Diamniotic Dichorionic twins

Multiple Pregnancy

Normal imaging milestones 1- Double decidual sac Sign (DDSS) : is the

earliest sign of IUP seen by 4-5 wks from LMP by TV U/S formed of two echogenic rings and it’s where the fertilized ovum implants itself into the decidualized endometrium and formed of :

- Decidua Parietalis : outer ring lining uterine cavity- Decidua capsularis : Inner ring covering the free

margin of the G.S - Decidua Basalis : which the endometial base of

the sac

Decidua basalis + chorion frondosum = placenta

Yolk sac The presence of the yolk sac within the uterus

confirms IUP Seen by 5-6 weeks from LMP Round and echogenic Though the amnion develops embryologically

before the yolk sac the yolk sac is easier to identify by U/S

An imaging hint: the number of the yolk sacs is equal to the number of amnions so in multiple pregnancies count the yolk sacs to determine amnionicity.

The Embryo First seen as a focal thickening in the yolk sac “ Diamond ring Sign”: Embryo appears as the

echogenic (diamond) on top if the yolk sac (ring)

“ Double Bleb “ sign : yolk sac and amniotic sac and the embryo is in the amniotic sac

Distinct embryo with cardiac activity is seen by 5-6 weeks after LMP and a CRL of 5 mm is the discriminatory value of the presence of cardiac activity known as the “ 5 Alive “ rule.

Diamond ring Sign

Double Bleb Sign

Gestational sac

Mean sac diameter :- > 10mm you must see a yolk sac - > 18-20mm must see and embryo

Failing Pregnancy

Failure of any of the above milestones means a failed pregnancy

Some Imaging Protocols

When your measuring the Mean Sac Diameter (MSD) : measure only the sonolucent area and don’t include the echogenic chorionic rim, also measure the sac in three different diameters and take their average .

Crown – Rump length: is the most accurate means of dating pregnancy that should be used once an embryo is visible , remember not to include the yolk sac in your measurement and only measure the longest axis of the embryo

Normal 1st trimesteric Measurement MSD increases by about 1mm per day Sac Diameter should be about 1 cm longer than

CRL Cord length at this stage is almost as the

embryonic length Embryonic heart rate :

-< 6week = 110-115 bpm- By 8 weeks = 144-159 bpm- >9 weeks = 137-144 bpm- A rate <90 bpm is considered embryonic

bradycardia

Anomalies that could be detected in 1st trimester Anencephaly, acrania,

holoprosencephaly (don’t mistake normal rhombencephalon for a cystic brain mass)

“Mickey Mouse Appearance “

“ Bart Simpson” Appearence

Normal Rhombencephalon

Cystic hygroma

Abdominal wall defects but beware not to mistake them with physiologic hernia

1st trimester screening for aneuploidy Nuchal translucency : Should be strictly

< 3mm by standardized measuring techniques

Ductus venosus : there should a continuous forward flow all through the cardiac cycle in a triphasic wave form

Nasal bone should be identified as a separate bone from the skin

Questions you should be able to answer by the end of your scan

Is there an IUP? Is there a definite ectopic pregnancy or

suspicious sign of it? How many gestations are there? If there are multiple sacs what is the

chorionicty and amnionicity? Is there a yolk sac? Is there an embryo?

Clinical implications

Human chorionic gondatropin (hCG) : Nomral pregnancy leads to increase in

levels of hCG Discriminatory level of hCG can be very

useful in the triage of pain and bleeding in early pregnancy .

If hCG level is > 2000 expect to see IUP

Triage decision tree :

1- Empty uterus with hCG > 2000 D.D includes ectopic pregnancy and complete miscarriage , if there are clinical signs of ectopic pregnancy you have to choose between methotrexate injection v.s surgery but if the patient is stable with no U/S features of ectopic pregnancy follow up with serial hCG titring and U/S

2- Empty uterus with hCG <2000 your differential diagnosis includes ectopic pregnancy , miscarriage and normal early pregnancy