First Trimester

Multiple events occur in 1st trimester

• Ovulation with resultant corpus luteum in ovary

• Fertilization of ovum → zygote

• Zygote cleavage → blastocyst → embryoblast + trophoblast

• Implantation → intrauterine sac-like structure

• Embryonic development, organogenesis

• Development of placenta, umbilical cord

Embryoblast cells form embryo and amnion

• Bilaminar embryonic disc forms when embryoblast splits into epiblast and hypoblast

• Trilaminar disc develops by process of gastrulation, which moves cells to different locations

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    Creates 3 primary germ layers of ectoderm, mesoderm, endoderm

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    Determines axes of body

Embryonic disc lies between amnion and yolk sac

• Disc folds on itself to form a tube surrounded by amnion

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    Yolk sac is pinched off as embryonic disc sides come together to form tube

• Series of tubes within tubes elongate, bud, rotate, and form all major organs by end of 13th week

Trophoblast cells give rise to membranes and placenta, not embryo proper

Gestational sac initially covered in chorionic villi, atrophy of those adjacent to uterine cavity → chorion laeve

Chorionic villi adjacent to implantation site develop into mature tertiary villi

• Tertiary villi contain fully differentiated blood vessels for gas exchange

• This part of chorion increases in thickness and echogenicity → chorion frondosum

Maternal endometrial cells differentiate into decidual cells

Chorion frondosum + maternal decidua basalis = placenta

Embryonic disc lies between amnion and yolk sac

• Embryo initially connected to chorion by connecting stalk

• Umbilical cord forms from incorporation of connecting stalk, allantois, vitelline duct

Earliest visible structure is an intrauterine sac-like structure

• Measure internal diameter (i.e., fluid component only, not echogenic wall)

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    Average of 3 orthogonal planes = mean sac diameter (MSD)

• Sac-like structure is chorion; commonly referred to as gestational sac (GS)

• Next visible structure is yolk sac (YS) inside GS

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    Round, distinct wall, normal size ≤ 6 mm

• Next visible structure is embryo

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    1st visible as thickening of YS wall

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    As amnion expands double bleb sign seen with embryo inside amnion, YS attached to embryo

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    Abdominal wall closure → YS separation from embryo

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      Embryo inside amnion, YS outside amnion

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    Embryo elongates, develops clear cranial (crown) vs. pelvic (rump) polarity, limb buds appear

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    At 10 weeks gestation, embryo → fetus

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    Organogenesis complete by 13 weeks

• Measure crown rump length (CRL) as longest axis of embryo; do not include YS