Chapter 49: The Early Embryonic Stage of the First Trimester

embryologists state time in ____, also known as ____.

conceptual age, embryologic age

conception is ___.

first day of pregnancy

clinicians and sonographers use ____ to date pregnancy.

gestational age (menstrual age)

____ is the beginning of the gestation.

first day of last menstrual period

for 12 days after conception during the implantation process when the ovum becomes fertilized, the conceptus is called a ___.

zygote

from the time of implantation until the end of the 10th week menstrual age, the conceptus is called a(n) ___.

embryo

after the first 10 weeks, the embryo is called a ____.

fetus

during a 28 day menstrual cycle, what happens at day 14?

a mature ovum is released (ovulation)

the ovum is swept into the distal fallopian tube via ___.

fimbriae

the follicle that released the mature ovum hemorrhages and collapses forms the ___.

corpus luteum

the corpus luteum secretes ___ and ___.

progesterone and estrogen

the zygote undergoes rapid cellular division to form the ___.

morula

the morula is formed of how many different cells?

16

once the morula is formed, further cell proliferation brings the morula to the ____ stage.

blastocyst stage

the blastocyst contains ____ and the inner cell mass that forms the embryo.

trophoblastic cells

the trophoblastic cells secrete ___.

human chorionic gonadotropin (hCG)

absorbed within the tubes and stimulates maternal pregnancy responses

human chorionic gonadotropin (hCG)

what does hCG cause the endometrium to do?

converts it to decidua

glycogen-rich mucosa that noursihes the early pregnancy

decidua

the blastocyst typically enters the uterus ____ days after fertilization.

4-5 days

implantation into the uterine decidua is completed within how many days after fertilization?

12 days

blood pools that form as maternal capillaries erode nourish the proliferating trophoblastic cells

lacunae

when implantation is complete, the trophoblast has formed ___, which initially encircle the early gestational sac.

primary villi

after implantation is complete and the trophoblast has formed primary villi, within the conceptus, the inner cell mass matures into the…

bilaminar embryonic disk, the future embryo, and the primary yolk sac

at approximately 23 days of menstrual age, the ____ is pinched off by the extraembryonic coelom.

primary yolk sac

at approximately 23 days of menstrual age, the primary yolk sac is pinched off by the extraembryonic coelom, forming the ____.

secondary yolk sac

which is the yolk sac seen sonographically throughout the first trimester?

secondary yolk sac

gestational sac should be visible by TV ultrasound at ____.

5 weeks

the yolk sac may be visible from ____ throughout the first trimester.

5.5 weeks

the embryo is usually seen at ___.

6 weeks

the initial heartbeat occurs at approximately ___.

5-6 weeks

embryo’s appearance changes from a ___ to a ____.

flat, disk-like configuration to a C-shaped structure

the CRL is developing rapidly and measures ____ by the end of the ___ week.

35mm; 10th

the last 3 weeks of the first trimester constitute the beginning of the ____.

fetal period

embryonic phase happens between weeks ___ and ___.

4 and 10

CRL should correspond to the ___.

gestational age

gestational sac size and hCG levels increase ____ until 10 menstrual weeks.

proportionally

at 10 menstrual weeks, the mean sac diameter is approximately ____.

45 mm

normal gestational sac can be seen beta hCG is _____ miU/mL.

1000-2000

an hCG of ____ with no IUP present indicates ectopic pregnancy.

2000 or greater than 3000

hCG should ____ every ___ days.

double, two

normal IUP greater than 7 weeks demonstrates…

doubling of quantitative maternal serum hCG levels every 3.5 days or increase of 66% in hCG levels within 48 hours

abnormal pregnancies demonstrate ___ hCG levels relative to gestational sac development.

low

hCG levels fall before…

spontaneous expulsion of nonviable gestations

at 3 menstrual weeks, hCG levels should range from…

0-5 miU/mL

at 4 menstrual weeks, hCG levels should range from…

5-426 miU/mL

at 5 menstrual weeks, hCG levels should range from…

18-7340 miU/mL

at 6 menstrual weeks, hCG levels should range from…

1080-56,500 miU/mL

at 7-8 menstrual weeks, hCG levels should range from…

7650-220,000 miU/mL

at 10-12 menstrual weeks, hCG levels should range from…

25,700-288,000 miU/mL

what should happen to hCG levels around 9-10 weeks?

plateau and subsequently decline

in pregnancies where the fetus is trisomy 21, what occurs with the hCG levels?

plateau later and fall much slower

increased hCG levels can be used as screening marker for ____ during ____.

down syndrome during first and second trimesters

insulin-like growth factor produced by trophoblastic (placental) cells during pregnancy

pregnancy-associated plasma protein (PAPP-A)

PAPP-A is involved in….

proliferative growth process such as bone and tissue formation

maternal serum PAPP-A ____ with advancing gestation.

increases

amniocentesis can be performed ____.

after 16 weeks

what is CVS?

chorionic villi sampling - gathers cells/blood from the chorion

what are the major components of routine first trimester examination?

uterus and adnexa evaluated for presence of GS, measurements of embryo and/or sac, presence or absence of cardiac activity, fetal number and chorionicity assessed, uterus/adnexal structures/CDS evaluated, NT may be measured, fetal anatomy survey

circumferential echogenic rim seen surrounding gestational sac represents…

trophoblastic tissue (decidual reaction)

describe the sonographic appearance of a typical gestational sac.

round or oval, smooth contour, echogenic wall

what should the position of a typical gestational sac be?

fundal or middle portion of the uterus; a center position relative to endometrium

implantation in lower uterine segment may be associated with ___ or ___.

placenta accreta or placenta previa

lacunae growing through the myometrium is associated with ___.

placenta accreta

GS implants too low and placenta covers the cervix

placenta previa

yolk sac should be seen when the mean sac diameter (MSD) measures ____.

>6 mm

the yolk sac is measured ___ to ____.

inner to inner

embryo should be seen with MSD measures ___.

>18 mm

gestational sac grows at a predictable rate of ____ in early pregnancy.

1 mm/day

the secondary yolk sac, what we see on ultrasound, is routinely visualized between _____ and ____ weeks of gestation.

5 and 5 ½

normal diameter of yolk sac should not exceed ___.

6 mm

growth rate of yolk sac has been reported to be approximately ______ of growth of MSD when MSD measures _____ and ____ of growth of MSD through first trimester.

0.1 mm/ml; <15 mm; 0.03 mm/ml

enlarged yolk sacs may have ___ outcomes.

ominous

two big circles seen side by side in the first trimester

double bleb sign

number of yolk sacs is consistent with the number of ___.

amnion membranes

in twin pregnancies, one YS =

monochorionic, monoamniotic

in twin pregnancies, two YS =

monochorionic, diamniotic OR dichorionic/diamniotic

typically, yolk sac resorbs and is no longer seen sonographically by ____.

12 weeks

where may persistent yolk sac be visualized?

at placental umbilical cord insertion where amniotic and chorionic membranes fused

the amnion and chorion will fuse around ___.

14 weeks

the embryonic heartbeat should be seen in a viable embryo when the CRL is ____.

greater than 4 mm

the yolk sac has essential functions in embryonic development, including…

provision of nutrients, hematopoiesis, development of embryonic endoderm (which forms the primitive gut)

as embyronic folding continues, what is formed that creates the yolk stalk?

the embryonic head, caudal portions, and lateral folds form that creating the constriction/narrowing between embryo and yolk sac

during embyronic folding, the dorsal aspect of the yolk sac is incorporated with…

the embryonic gut

the yolk stalk, connecting stalk, and allantois are brought together by the expanding amnion that covers the three structures forming the ____.

umbilical cord

the embryonic spine develops from the ___.

ectoderm

the embryonic spine initially evolves from the ___, which closes about the 6th week of gestation.

primitive neural tube

developing spine may be visualized sonographically as ____ at ___ weeks of gestation.

parallel echogenic lines, 6 weeks

by 8 weeks, there are three primary vesicles seen within the fetal head. the ___, ___, and ____.

presencephalon, mesencephalon, and rhombencephalon

the forebrain =

prosencephalon

the midbrain =

mesencephalon

the hindbrain =

rhombencephalon

cystic structure seen within the posterior aspect of the embryonic cranium between the 8th and 10th week of gestation

rhombencephalon

the choroid plexus should be seen by ___ weeks.

9-10 weeks

the lateral ventricles are seen filled by the ___.

choroid plexuses

in limb development, what develops first?

arms

calcification of clavicle begins at approximately ___.

8 weeks

after the ossification of the clavicle, ossification of __ follows.

mandible, palate, and vertebral column

frontal cranial bones begin to calcify at 9 weeks, followed by ____.

long bones

the anterior abdominal wall is developed by 6 weeks of gestation from fusion of…

four ectomesodermal body folds