Fetal Development and Fetal Growth Assessment for Nurses

You assess fetal growth at every prenatal visit, and the milestones tell you whether development is on track. Know the timeline from fertilization through term, the fetal structures and their jobs, and the growth assessments cold. They are how you catch a problem early enough to act.

Fetal Development

Fertilization

Fertilization is the union of ovum and spermatozoon at the ampullary portion of the fallopian tube, the usual site.

• A mature ovum can only be fertilized within 24 to 48 hours after release.
• The spermatozoa have a functional life of only 48 to 72 hours.
• Fertilization is most likely 72 hours after intercourse.
• Peristaltic movements and the cilia of the tube propel the fertilized ovum along.
• Sperm reach the cervix within about 90 seconds and the outer fallopian tube within 5 minutes.
• Once a sperm penetrates the ovum, the ovum changes composition to become impermeable to other sperm.
• After penetration, the chromosomal material of ovum and sperm combine to form a zygote.
• Three factors determine whether fertilization happens: maturation of both sperm and ovum, the sperm's ability to reach the ovum, and the sperm's ability to penetrate the cell membrane.

Implantation

• The zygote migrates toward the uterus over 3 to 4 days, propelled by muscular contractions of the tube.
• Mitotic cell division (cleaving) begins after 24 hours, averaging one division every 22 hours.
• By the time it reaches the uterus, it has 16 to 50 cell divisions and is called a morula for its bumpy appearance.
• The morula floats freely in the body of the uterus for 3 to 4 days and becomes a blastocyst.
• The blastocyst attaches to the endometrium. This is implantation, occurring 8 to 10 days after fertilization.
• Apposition (the blastocyst brushing against the endometrium) is the first step, then adhesion as it attaches to the surface, then invasion as it settles into the endometrial folds.
• On the day of implantation, the woman may notice slight vaginal spotting from the capillaries disrupted by the implanting blastocyst.
• Once implanted, the zygote becomes an embryo.

Fetal Structures

Decidua

• Under the influence of hCG, the corpus luteum keeps the endometrium thickening. Instead of sloughing off as in a normal cycle, it becomes the decidua.
• The decidua has three parts: basalis, capsularis, and vera.
• The decidua basalis is the innermost layer, directly under the embryo.
• The decidua capsularis encapsulates the trophoblast's surface.
• The decidua vera is the remaining uterine lining and sheds as the lochia.
• The decidua vera and capsularis eventually fuse as the embryo enlarges.

Chorionic Villi

• On the 11th or 12th day, chorionic villi begin forming from miniature villi protruding from a single cell layer, starting placental formation.
• The villi have a central core with fetal capillaries and a double layer of trophoblast cells.
• The syncytial (outer) layer produces placental hormones: hPL, hCG, estrogen, and progesterone.
• The Langhans' layer, the middle layer, protects the embryo and fetus from infectious disease. It functions as early as 12 days' gestation and disappears at the 20th to 24th week, leaving the fetus more susceptible to infection.

Placenta

The placenta (Latin for "pancake," for its shape) develops from trophoblast tissue and serves the fetus in several ways.

• It transports nutrients (glucose, amino acids, vitamins, minerals, fatty acids, water) and oxygen from maternal blood to the fetus.
• Placental osmosis is selective. It is impermeable to some harmful substances and keeps them out of fetal circulation.
• The syncytial layer produces hormones that benefit mother and fetus. Human chorionic gonadotropin is the first placental hormone produced and keeps the corpus luteum making estrogen and progesterone to support the pregnancy.
• Estrogen aids uterine growth and mammary gland development for lactation.
• Progesterone maintains the uterine lining and reduces uterine contractility to prevent preterm labor.
• Human placental lactogen promotes mammary growth and lactogenic properties for maternal lactation.

Amniotic Membranes

• The smooth portion of the chorionic villi becomes the chorionic membrane, forming the sac that holds amniotic fluid.
• The amniotic membrane forms under the chorion, so the two look like one.
• The amniotic membrane produces the amniotic fluid and the phospholipids that trigger prostaglandins, the hormone that initiates uterine contractions.

Amniotic Fluid

• The normal amount is 800 to 1000 mL.
• It protects the fetus from trauma and pressure on the mother's abdomen, regulates fetal temperature, and aids muscular development by letting the fetus move freely.
• It also protects the umbilical cord from trauma and pressure, protecting the fetal oxygen supply.

Umbilical Cord

• The amnion and chorion form the umbilical cord, which connects the embryo to the chorionic villi of the placenta.
• Its job is to carry oxygen and nutrients from placenta to fetus and return waste from fetus to placenta.
• It is made of a gelatinous mucopolysaccharide, Wharton's jelly, that protects the vessels from trauma.
• The cord has one vein, which carries blood from the placenta to the fetus, and two arteries, which carry blood from the fetus to the placenta.

Fetal Milestones

4th Week of Gestation

• Spinal cord forms and fuses at the midpoint.
• Head folds forward and is prominent.
• The back is bent, bringing the head almost to the tail.
• A prominent bulge appears that will become the heart.
• Lateral wings fold forward and fuse at the midline.
• Arms and legs are budlike.
• Eyes, ears, and nose are barely recognizable.

8th Week of Gestation

• Organogenesis is complete.
• The heart has a septum and valves and beats rhythmically.
• Arms and legs have developed.
• Facial features are noticeable.
• Genitals begin to form but are not yet recognizable.
• The fetal intestine is growing rapidly.
• Ultrasound shows a gestational sac, confirming pregnancy.

12th Week of Gestation

• Toes and fingers have nail beds.
• Faint fetal movements begin.
• Early reflexes are present.
• Tooth buds are forming.
• Bone ossification centers begin to form.
• Genitals are recognizable.
• Urine secretion begins but is not yet evident.
• Heartbeat is detectable by Doppler.

16th Week of Gestation

• An ordinary stethoscope can detect the fetal heartbeat.
• Lanugo begins to form.
• The pancreas and liver are forming.
• Urine is present in the amniotic fluid.
• The fetus begins to swallow amniotic fluid.
• Ultrasound can determine fetal sex.

20th Week of Gestation

• The mother can sense spontaneous fetal movements.
• Hair forms on the head and eyebrows.
• The upper intestine contains meconium.
• Brown fat forms behind the kidneys, sternum, and posterior neck.
• Vernix caseosa forms and covers the skin.
• Passive antibody transfer begins.
• Sleep and activity patterns are evident.

24th Week of Gestation

• Lung surfactant begins to develop.
• Meconium is present in the rectum.
• Eyebrows and eyelashes are distinguishable.
• Eyelids can open.
• Pupils react to light.
• The fetus reaches the age of viability and could survive externally with modern intensive care.
• Responds to sudden sounds.

28th Week of Gestation

• Surfactant is demonstrable in the amniotic fluid.
• Alveoli begin to mature.
• Testes descend into the scrotal sac.
• Retinal blood vessels form but are highly susceptible to damage.

32nd Week of Gestation

• Subcutaneous fat is deposited.
• The fetus responds to sounds outside the mother's body with movement.
• Active Moro reflex is present.
• Iron stores begin to develop.
• Fingernails begin to grow.

36th Week of Gestation

• Iron, carbohydrate, calcium, and glycogen stores deposit in the body.
• Additional subcutaneous fat is deposited.
• One or two creases are present on the sole of the foot.
• Lanugo begins to diminish.
• Some babies turn to a vertex presentation.

40th Week of Gestation

• The fetus kicks actively and hard enough to cause discomfort.
• Fetal hemoglobin is converting to adult hemoglobin.
• Vernix caseosa is fully formed.
• Fingernails extend to the fingertips.
• Sole creases cover at least two-thirds of the surface.

Fetal Growth Assessment

Estimating Fetal Growth

McDonald's Rule

• McDonald's rule measures fundal height from the symphysis pubis.
• Place the woman supine and measure from the symphysis pubis to the uterine fundus.
• The distance in centimeters matches the week of gestation between the 20th and 31st weeks.
• At 12 weeks, the fundus is at the symphysis pubis.
• At 20 weeks, the fundus is at the umbilicus.
• At 36 weeks, the fundus is at the xiphoid process.

Fetal Movement

• Quickening, the first fetal movement the mother feels, usually starts at 18 to 20 weeks.
• A healthy fetus moves at least 10 times a day.
• Sandovsky method: have the woman lie recumbent after a meal and record fetal movements for one hour. The fetus normally moves at least twice every 10 minutes, or 10 to 12 times an hour. If fewer than 10 movements occur in an hour, she repeats for the next hour.
• Cardiff or "Count-to-Ten" method: the woman records the time interval to feel 10 fetal movements within 60 minutes.

Fetal Heart Rate

Rhythm Strip Testing

• Normal fetal heart rate is 120 to 160 beats per minute.
• Rhythm strip testing checks for a good baseline rate and variability.
• Variability is categorized as absent (none apparent), minimal (extremely small fluctuations), moderate (6 to 25 beats per minute), and marked (over 25 beats per minute).
• The woman stays in a fixed position for 20 minutes.

Nonstress Test

• Measures the fetal heart rate response to fetal movement.
• The woman is attached to a fetal heart rate and uterine contraction monitor and pushes a button whenever she feels movement.
• Normally the heart rate rises about 15 beats per minute and stays elevated for 15 seconds with movement.
• The test runs 10 to 20 minutes.
• Reactive: two accelerations lasting 15 seconds after movement.
• Nonreactive: no accelerations after movement, or no movement at all.
• A nonreactive test leads to a contraction stress test or biophysical profile.

Contraction Stress Testing

• Assesses the fetal heart rate against uterine contractions.
• The woman is attached to an external uterine contraction and fetal heart rate monitor and rolls a nipple between her fingers and thumb to produce contractions.
• Within a 10-minute window, three contractions lasting 40 seconds or longer must occur.
• Negative (normal): no decelerations during contractions.
• Positive (abnormal): late deceleration at the end of and after a contraction.

Ultrasonography

• Measures the response of sound waves against solid objects.
• It can diagnose pregnancy at 6 weeks' gestation, confirm the placenta's presence, size, and location, confirm fetal growth, detect gross anomalies, establish fetal sex, and determine fetal presentation and position.
• The woman needs a full bladder. Have her drink a full glass of water every 15 minutes for the 90 minutes before the procedure, until it starts.
• It predicts fetal maturity by measuring the biparietal diameter of the fetal head.
• Placental grading: 0 (12 to 24 weeks), 1 (30 to 32 weeks), 2 (36 weeks), and 3 (38 weeks).
• It also estimates amniotic fluid volume, another measure of fetal health.

Electrocardiography

• Fetal ECG can be recorded as early as the 11th week.
• It is inaccurate before the 20th week because fetal electrical conduction is still weak.

Magnetic Resonance Imaging

• MRI has no harmful effects on mother or fetus and is now a preferred fetal assessment technique.
• It can diagnose complications like ectopic pregnancy and trophoblastic disease (H-mole), since fetal movement can obscure findings later in pregnancy.

Maternal Serum Alpha Fetoprotein

AFP is found in amniotic fluid and maternal serum and is produced by the fetal liver.

• MSAFP levels begin rising at 11 weeks' gestation and climb steadily to term.
• MSAFP is abnormally high with a spina bifida or abdominal defect.
• MSAFP is low with a chromosomal defect such as Down syndrome.
• MSAFP is assessed at the 15th week and detects 85% to 90% of neural tube defects and 80% of Down syndrome.

Amniocentesis

Amniocentesis aspirates amniotic fluid from the pregnant uterus for examination.

• Done between the 14th and 16th weeks, when amniotic fluid is plentiful.
• Have the woman void, then place her supine.
• Attach fetal heart rate and uterine contraction monitors and take blood pressure and fetal heart rate.
• Run an ultrasound first to locate the fetus, a pocket of amniotic fluid, and the placenta.
• Apply antiseptic to the abdomen and inject local anesthetic.
• Tell her she may feel pressure as the needle goes in, but do not have her take a deep breath and hold it.
• About 15 mL of amniotic fluid is aspirated.
• The fluid is analyzed for AFP, bilirubin, chromosome analysis, color, fetal fibronectin, inborn errors of metabolism, lecithin-sphingomyelin ratio, phosphatidylglycerol, and desaturated phosphatidylcholine.

Biophysical Profile

• Combines five parameters into one assessment.
• Fetal heart rate and breathing measure short-term central nervous system function.
• Amniotic fluid volume measures long-term placental function.
• It is more accurate than any single assessment method.
• The score ranges from 2 to 10, with 10 the highest. A score of 8 to 10 means the fetus is doing well, 6 is suspicious, and 4 means the fetus may be in jeopardy.
• It works like Apgar scoring and is often called the fetal Apgar.