Pregnancy & Embryonic Development

FHA Final Exam Review // Spring 2025

What does prenatal mean?

Prior to birth

Pre-embryonic development

• Fertilization to implantation

• Approx 2 weeks

Embryonic development

Implantation to the end of the 8th week of pregnancy

Fetal development

9th week of pregnancy to birth

Prenatal development is known as?

Gestation period (nine lunar months or 40 weeks)

Prenatal development is divided into…

Three, 3-month trimesters:

• First trimester

• Second trimester

• Third trimester

First trimester

• Rudiments of all organs appear

• Most dangerous period in prenatal life

  • Only 40% of conceptions survive

• 1 to 12 weeks

Second trimester

• Further development of organs and organ systems

• Fetus looks like a human

• Fetus is covered by the amnion

• Fetus grows faster than the placenta

Third trimester

• Phase of rapid growth

  • Much growth in size and weight

• Most organ systems become functional

  • Most organs are formed but still maturing

  • Lungs are last to develop

Four events within the 1st trimester:

• Cleavage (sequence of cell divisions)

  • A blastocyst forms

• Implantation (implantation into endometrial lining)

• Placentation (formation of the placenta)

• Embryogenesis (development of the embryo)

First trimester - Implantation

• Gastrulation and germ layer formation

  • The ectoderm, mesoderm, and endoderm are collectively known as the germ layers

  • Each layer will form specific tissues and organs of the body

First trimester - Placentation

• Placenta develops

• Placental circulation

  • Blood flows from the fetus to the placenta in the paired umbilical arteries (away from the fetal heart)

  • Blood returns via a single umbilical vein (fresh oxygenated blood to fetus)

How does the maternal blood supply nourish the baby?

NOT through mixing of blood, but by diffusion of nutrients; oxygen, etc.

Amnion

Innermost membrane within placental sac

First trimester - Embryogenesis

• At about 4-8 weeks post-fertilization, organs begin to form

• This is called organogenesis

Changes in the uterus during gestation

• Uterus will increase in length from 7.5 cm to 30 cm

• Contains almost 5 L of fluid

• the uterus and contents weigh about 22 pounds

• Maternal abdominal organs are pushed out of their normal positions — these organs remain functional but are displaced

What is labor?

A series of strong, rhythmic uterine contractions

What is the goal of labor?

Parturition (expulsion of the fetus)

What are the 3 stages of labor?

• Dilation stage

• Expulsion stage

• Placental stage

What hormone stimulates contraction of the uterus?

Oxytocin

Labor - Dilation Stage

• Cervix dilates, the fetus is pushed by muscular contractions into the cervical canal, and the amnion ruptures — water breaking

Labor - Expulsion Stage

• the movement of the fetus through the cervical canal and vagina

• Delivery can be helped by episiotomy (cut between vagina and anus) or cesarian section

Labor - Placental Stage

Ejection of the placenta (afterbirth)

Placenta previa

A condition in which the placenta attaches low in the uterus and partially or completely covers the cervix, the opening of the uterus

The umbilical cord physically connects what?

The embryonic/fetal body with the placenta

Where is the placenta located?

Physically adjacent to the uterine wall in the mother’s body

The umbilical blood vessels enter and exit the fetal body at the…

Umbilicus (belly button)

Fetal bypasses that exist in the fetal heart and liver

In the fetal heart, the blood largely bypasses the fetal lungs. Remember that the fetus is in a liquid environment. There is no mechanical breathing taking
place in the fetal lungs. Gas and nutrient exchange is taking place at the interface between the uterine wall and the placenta. It is not necessary in the fetal body to send a significant amount of blood towards the lungs. That is why these bypasses exist. After birth, however, these bypasses should seal off. This then forces blood to enter into the newborn’s lungs

Premature Labor

• Labor that begins before the fetus has completed normal development

• Fetuses born <36 wks (birth wt > 1 kg) are considered premature

  • Generally survive with care

• Fetuses born <24 wks (birth wt below 600 g) generally die

  • Respiratory/cardiovascular/urinary systems have not developed enough to support life

Neonatal period

Period from birth to 1 month

Events that occur in the neonate during the neonatal period:

• Lungs fill with air

• Blood circulation changes with the closing of the ductus arterioles and the foramen ovals of the heart

• Heart rate drops from 110-190 bpm to about 80-130

• Breathing rate commences at 30-60 breaths per minute

• Kidney’s filter the infant’s own blood

• Digestive system becomes active

• Metabolic rate is increased to maintain warmth for a few days after birth