Embryology, Prenatal, Developmental Basics

Embryology - 1st Week Stages

1. ovulation

2. fertilization

3. cleavage

4. blastulation

ovulation cycle

• hypothalamus released GnRH to stimulate anterior pituitary gland

• LH released and estrogen levels increase

• oocyte popped from ampulla (done w/ meiosis I, stuck in metaphase II)

fertilization

sperm fusion with nuclear chromosomes released into oocyte creating diploid (2n) zygote

cleavage

diploid cell divides:

• 1 cell stage

• 2 cell stage

• 4 cell stage

• 8 cell stage

• 16 cell stage = morula

blastulation

cells migrate to line edge and group to one side (outer and inner cell mass), creating blastocyst

trophoblast

• outer cell mass

• becomes placenta

embryoblast

• inner cell mass

• become bilaminar disk - actual embryo

gastrulation

• cells begin migration towards primitive streak

• move through the groove and hypoblast to replace layers

  • hypoplast = endoderm

  • mesoderm created

neurulation

• proliferation of ectodermal cells to create neural plate and folding into neural tube

• process begins at day 17

Neural Tube Timing

• Day 21: structure created

• Day 24: anterior closure

• Day 26: posterior closure

• CLOSED at 4 weeks

embryonic age

measures from moment of fertilization/conception

gestational age

measures from first day of last menstrual period (embryonic age + 2 weeks)

All or Nothing Stage

• first two weeks after conception

• 1-4 weeks GA

• exposures during this period may cause loss

Embryonic Stage

3 - 11 weeks GA

Fetal Stage

12 weeks GA until end of pregnancy

major morphologic abnormalities

typically occur with exposures between 5 - 10 weeks GA

functional defects and minor morphologic abnormalities

typically occur with exposures past 10 weeks GA

Age of viability

• generally considered 24 - 30 weeks GA

• roughly 50% survival rate

• may differ depending on state definitiions

Palate closure

11 weeks GA

1st trimester

0 weeks - 13 weeks 6 days

2nd trimester

14 weeks - 27 weeks 6 days

3rd trimester

28 weeks - term

Pre-term birth

GA <37 weeks

Full-term birth

GA between 37 - 42 weeks

Post-term birth

GA >42 weeks

major anomaly

congenital anomaly requiring medical/surgical intervention

minor anomaly

congenital developmental difference without medical impact

malformation

development intrinsically abnormal (starts wrong)

disruption

development starts normal, but interrupted due to interference with normal development

• causes breakdown of structures or tissue loss

deformation

development starts normal, but mechanical stress produces abnormal shape/restriction of growth/movement

syndrome

recognizable pattern of anomalies assumed to be causally related

sequence

multiple/pattern of anomalies caused by mechanistic failure of development leading to a domino effect

association

collection of anomalies that co-occur at a higher than expected statistical chance

intracervical insemination (ICI)

sperm inserted as close to cervix as possible with syringe (must track ovulation)

intrauterine insemination (IUI)

washed, concentrate sperm placed directly into the uterus (done 1 day after LH surge)

in vitro fertilization (IVF)

1. ovarian stimulation

2. egg retrieval with ultrasound guided needle

3. fertilization

4. embryo transfer/freezing

intracytoplasmic sperm injection (ICSI)

fertilization of ovum with one singular sperm (usually done with IVF)

Pre-implantation genetic testing

analysis of genetic composition of IVF embryo prior to implantation

• either at 8 cell stage or after blastocyst formation

PGT-M

• looks for monogenic disease

• disease causing mutation must be known

• direct and linage analysis done to account for allele drop out

PGT-A

• check for aneuploidy

• unable to rule out mosaicism (5-20% embryos), small losses or gain, or structural changes

PGT-SR

• check for structural rearrangements of chromosomes

• better resolution than PGT-A

PGT-P

• looks at polygenic risk

• controversial

1st Ultrasound/Dating Scan

done between 6-10 weeks GA to check for heartbeat and dating purposes

Formal Anatomy Scan

ultrasound done in 2nd trimester (18 - 22 weeks GA) to assess anatomy of fetus

anencephaly

incomplete forebrain development (lethal)

encephalocele

protrusion of brain and membranes through occipital defect of the neural tube

Spina Bifida

most common neural tube defect

spina bifida occulta

gap in vertebrae with minimal change, dimple or tuft of hair on skin

meningocele

sac of CSF through gap in spine, spinal nerves do not extend into the sac

myelomeningocele

sac of CSF through gap in spine including spinal nerves

General Population Risk for NTD

1 in 1,000 (0.1%)

Risk for NTD w/ Valproic Acid Exposure

10 in 1,000 (1%)

1 prior fetus with NTD → Recurrence Risk

1.5 - 3%

2 prior fetuses with NTD → Recurrence Risk

5 - 6%

General Folic Acid Intake

0.4 mg daily

Folic Acid Intake with FHx of NTD

4 mg daily

Pierre-Robin Sequence

1. small mandible

2. tongue displaced posteriorly

3. tongue blocks midline palata fusion

   • may block larynx formation

Potter Sequence

1. oligohydramnios (from renal problems)

2. restricted growth

3. underdeveloped lungs

4. facies

Potter Sequence Pneumonic

Pulmonary hypoplasia

Oligohydramnios

Twisted face

Extremity defects

Renal failure

VACTERL Association

• V = vertebral

• A = an*l atresia

• C = congenital heart defect

• TE = tracheoesophageal fistula

• R = renal anomalies

• L = limb anomalies

  • clinical diagnosis with 3+ features

Dichorionic/diamniotic (di/di) Twins

• most common type of twin

• the only type dizygotic twins can be

• dizygotic 80-90%

• occurs with morula division or dual fertilization

Twins - chorion

number of placentas

Twins - amnion

number of amniotic sacs

Monochorionic/Diamniotic (mono/di) Twins

• 100% monozygotic

• occurs with blastocyst division

Monochorionic/Monoamniotic (mono/mono)

• most risk type of twin pregnancy

• occurs with implanted blastocyst division

Twin-Twin Transfusion Syndrome (TTTS)

• affects 10-20% of monochorionic twins

• recipient twin = polyhydramnios, heart enlargement, hydrops

• donor twin = oligohydramnios, organ failure, growth restriction

• can be treated with fetal surgery (laser ablation)

MS-AFP Screening

• measures levels of AFP in mother’s blood

• can be done during amniocentesis

• high levels can indicate NTDs or AWDs

Second Trimester Screening

• maternal serum screening done between 15 - 21 weeks 6 days GA

• can include different markers (hCG, AFP, uE3, DIA)

cfDNA Counting Technology

• fragments compared to reference sequence

• pros: faster results, less reliant on fetal fraction

• cons: can’t differentiate between mom and fetus, can’t separate twin fetal sexes

cfDNA SNP Technology

• targeted sequencing of thousands of SNPs

• pros: can differentiate between mom and fetus, screens for triploidy and vanishing twin, provides separate fetal sexes

• cons: more reliant on fetal fraction, longer TAT

PPV for Turner Syndrome with NIPT

25%

ACOG Guidelines - Screening

screening and diagnostic testing for aneuploidies offered to ALL patients

ACMG Guidelines - Screening

NIPS over traditional screen for ALL patients for aneuploidy, SCAs, and 22q11.2 microdeletion

ACOG Guidelines - Carrier Screening

carrier screen offered to ALL pregnant people/those planning pregnancy

• SMA, CF, with CBC (if abnormal → hemoglobinopathies)

• Fragile X based on FHx

• Tay-Sachs, Canavan disease, and familial dysautonomia for AJ populations

CF Carrier Screening Considerations

• most carrier screens do NOT completely sequence CFTR

• ACMG recommends 23 most common mutations tested

SMA Carrier Screening Considerations

• severity of condition dependent on number of SMN2 genes

• silent carriers may be missed

Hemoglobinopathies Carrier Screening Considerations

• silent carriers for Alpha-Thalassemia may be missed

• ACOG recommend CBC then electrophoresis then DNA test

Fragile X Carrier Screening Considerations

• recommended for FHx of fragile X or ID

• those with unexplained primary ovarian insufficiency <40y

Tay Sachs Carrier Screening Considerations

• consider enzyme testing for those of non-AJ descent

• hexosaminidase activity is higher sensitivity

ACMG Guidelines - Carrier Screening

• all pregnant patients/those planning pregnancy should be offered tier 3 screening

• conditions with carrier frequency >1 in 200

  • about 113 conditions

Chorionic Villi Sampling (CVS)

• 10 - 13 weeks GA

• samples chorionic villi from placenta

• earliest diagnostic results

• increased miscarriage risk (1 in 200), limb reductions if done early

Chorionic Villi Sampling (CVS) Limitations

• possibility for confined placental mosaicism

• MCC

• can’t screen for AFP or methylation status

Amniocentesis

• 15 - 20ish weeks

• tests amniotic fluid

• lower miscarriage risk (1 in 500), can test for AFP

• chorion must be fused, risk for rupture

Percutaneous Umbilical Blood Sampling (PUBS)

• 17 weeks or later

• tests for fetal anemia and can provide blood transfusions

• may evaluate for mosaicism

• higher loss rate (2-3%)

ACOG Recommendation - Ultrasound Anomalies

CMA is a primary test for US anomalies

fetal loss

• 10-20% of pregnancies end in miscarriage

• 50-60% have chromosomal abnormalities

infertility

disease, condition, or status characterized by

• inability to adhere to a successful pregnancy

• need for medical intervention

Congenital Bilateral Absence of Vans Deferens (CBAVD)

• lack of vas deferens to transport sperm from testes to urethra

• sperm fine, but not present in ejaculate

• 100% AMAB w/ classic CF have CBAVD

Infertility - AFAB Factors

• endocrine disorders

• early menopause

• medications/toxic agents

• ovulation

• shape of uterus

• patency of tubes

Infertility - AMAB Factors

semen analysis

• volume

• count

• concentration

• motility

• morphology

• vitality

Klinfelter Syndrome (XXY)

• azoospermia/oligospermia

• microscopic testicular sperm extraction from tissue

Y Microdeletion

• causes 5-10% of male infertility

• deletion of AZF region

• impairs spermatogenesis

medication abortion

• done up to 11 weeks

• mifepristone followed by misoprostol (24-48hrs)

• 60% of all abortion in U.S.

Medication Abortion - Benefits

• avoids surgery and anesthesia

• privacy

• early in pregnancy

Medication Abortion - Cons

• requires follow-up visit

• takes multiple steps

1st Trimester Abortion Procedures

• manual vacuum aspiration → up to 12 weeks

• electric uterine aspiration → up to 14 weeks

Dilation and Cutterage (D&C)

• up to 13 - 14 weeks GA

• removes tissue from uterus

• outpatient procedure

Dilation and Evacuation (D&E)

• 14 - 24 weeks

• removes fetus from uterus (may be intact)

• prepare with misoprostol and laminaria

• requires sedation

Induction (Abortion)

• requires hospital admission

• fetus may be delivered intact

2 month milestones

• smiles at you

• reacts to loud sounds

• makes sounds other than crying

• watches you move

• head up during tummy time