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 chorionicitydepends 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 decidualizedendometrium 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