GESTATION MILESTONES AND RED FLAGS DEVELOPMENT AND RISKS GESTATION • Gestation is the process or period of developing of an embryo, and later fetus inside the womb between conception and birth. • It lasts about 280 days or 40 weeks. • Fetal growth and development can also be separated into three time periods: 1. Pre-embryonic (germinal/zygote) stage (first 2 weeks, beginning with fertilization) 2. Embryonic (weeks 3 through 8) 3. Fetal (from week 8 through birth) GERMINAL/ZYGOTE STAGE • This is the shortest stage of gestation and takes about 2 weeks. • It begins at conception with the sperm fertilising the egg to form a zygote. • The fertilized egg (zygote) travels through the fallopian tube over a period of about 8-9 days. • During this journey, the zygote divides many times, eventually creating two separate structures. One structure eventually becomes the embryo (and later, the fetus) and the other becomes the placenta • Eventually, the zygote turns into a blastocyst. The blastocyst arrives at the uterus and implants into the uterine lining. • If implantation is successful, the body immediately begins producing hormones to support a pregnancy. This also stops one’s menstrual cycles. EMBRYONIC STAGE • This stage lasts around 3-8 weeks. • The blastocyst begins to take on distinct human characteristics. It’s now called an embryo. • This is when the embryo begins to develop three primary germ layers called the ectoderm, mesoderm, and endoderm, which will eventually give rise to all the organs and tissues of the embryo. IMPLANTATION • This is the stage when the embryo attaches itself to the uterine wall. • Upon arrival to the uterus, the rapidly-changing embryo implants, or burrows, into the uterine lining. This happens 5-10 days after conception in almost all cases. GASTRULATION • Occurs in the 2nd week after fertilisation and lasts about 2 weeks • Gastrulation stage is the stage during prenatal development when the germ layers form and develop. • The germ layers are a layer of a group of cells that will designate which cells of the body will grow into certain organs and tissues. • A blastula, made up of one layer, folds inward and enlarges to create a gastrula. A gastrula has 3 germ layers--the ectoderm; Mesoderm; endoderm. • The final phase of gastrulation is the formation of the primitive gut that will eventually develop into the gastrointestinal tract. NEURULATION • Following gastrulation, the next major development in the embryo is neurulation, which occurs during weeks three and four after fertilization. • This is a process in which the embryo develops structures that will eventually become the nervous system. • It begins when neural plate forms from the ectoderm. • The neural plate then starts to fold inward until its borders converge. The convergence of the neural plate borders also results in the formation of a neural tube. Most of the neural tube will eventually become the spinal cord. The neural tube also develops a bulge at one end, which will later become the brain. ORGANOGENESIS • This is when organs develop within the newly formed germ layers. Most organs start to develop during the third to eighth weeks following fertilization. They will continue to develop and grow during the following fetal period. • The heart is the first functional organ to develop in the embryo. The primitive blood vessels start to develop in the mesoderm during the third week after fertilization. • Within days, the endocardial tubes grow. The tubes migrate toward each other and fuse to form a single primitive heart tube. • 21 or 22, the tubular heart starts to beat and pump blood, even as it continues to develop. • By day 23, it forms 5 distinctive regions which develop into the chambers of the heart and the septa (walls) that separate them by the end of the eighth week after fertilization WEEK FIVE • By week five after fertilization, the embryo measures about 4 mm (0.16 in.) in length and has begun to curve into a C shape. During this week, the following developments take place: • Grooves called pharyngeal arches form. These will develop into the face and neck. • The inner ears begin to form. • Arm buds are visible. • The liver, pancreas, spleen, and gallbladder start to form. WEEK SIX • By week six after fertilization, the embryo measures about 8 mm (0.31 in.) in length. During the sixth week, some of the developments that occur include: • The eyes and nose start to develop. • Leg buds form and the hands form as flat paddles at the ends of the arms. • The precursors of the kidneys begin to form. • The stomach starts to develop. WEEK SEVEN • By week seven, the embryo measures about 13 mm (0.51 in.) in length. During this week, some of the developments that take place include: • The lungs begin to form. • The arms and legs have lengthened, and the hands and feet have started to develop digits. • The lymphatic system starts to develop. • The primary prenatal development of the sex organs begins. WEEK EIGHT • By week eight — which is the final week of the embryonic stage — the embryo measures about 20 mm (0.79 in.) in length. During this week, some of the developments that occur include: • Nipples and hair follicles begin to develop. • External ears start to form. • The face takes on a human appearance. • Fetal heartbeat and limb movements can be detected by ultrasound. • All essential organs have at least started to form. 1ST TRIMESTER COMPLICATIONS • Since all major organ systems form during this time, the embryo is especially vulnerable to teratogens, or substances that can damage cells and cause abnormalities in development. • Teratogens can begin affecting the developing embryo as early as 10 to 14 days after conception. During embryonic development, there are periods when the developing organ systems show more sensitivity to teratogens. Specifically, if exposure to a teratogen occurs during the first 3.5 to 4.5 weeks of gestation, a neural tube defect, such as spina bifida or anencephaly, may result. • The first trimester combined screening test (maternal blood test + ultrasound of baby) can be done around this time. This test checks for trisomy 18 (Edward syndrome) and trisomy 21 (Down syndrome). • It is very important that during this time to prevent neural tube defects that the mother maintains a healthy diet with sufficient amounts of folic acid. • The risk of having a miscarriage is highest during the first trimester, and those risks can be minimized by taking prenatal vitamins and avoiding smoking, alcohol, and drugs, including some prescription drugs. • Other complications in the first trimester to be aware of include bleeding, • Hyperemesis gravidarum which is excessive vomiting, • Spontaneous abortions/miscarriages, • Ectopic pregnancies, • molar pregnancies FETAL STAGE • Finally, the third stage, called the fetal stage, takes place from the ninth week until birth and is characterized by rapid growth and development. During weeks 9 through 12, the central nervous system, GI system, and heart are fully developed. This is also when the heartbeats WEEK 9-15 • During weeks 9 to 15, the fetus’s reproductive organs develop rapidly. The external genitals of male and female fetuses are rather similar in appearance at first, but they will be clearly differentiated by week 12. At that point, the biological sex of the fetus can be determined with almost 100 percent accuracy using obstetric ultrasound. • Facial development continues. The eyelids form, the ears move toward their final position on the sides of the head, and tooth buds appear. • Fine, colorless hair called lanugo starts to grow on the fetus’s face. It will eventually cover most of the body until it is shed close to the time of birth. • The thyroid gland matures and starts producing thyroid hormones.The liver and pancreas also start producing their secretions. • The kidneys start functioning. The amniotic fluid the fetus swallows can now pass out of the body as urine. • The fetus is very active. This is the result of uncontrolled movements and twitches that occur as the muscles, brain, and nervous pathways develop. The fetus may move its limb, make a fist with its fingers, and make sucking motions with its mouth. Generally, brain activity can be detected during these early weeks of the fetal stage. WEEKS 16 TO 26 • The brain and sensory nerves develop to the point that the fetus has a sense of touch. This may lead to the fetus stroking its face, touching its limbs, and even sucking its thumb. • The eyes move to a forward-facing position, and the retinas develop. The ears also move to their final position, and the outer ears are now elevated above the surface of the head. Development of the middle ear and auditory nerve allows the fetus to hear. It can hear both internal sounds (such as the mother’s heartbeat) and external sounds (such as voices). The fetus may even be startled by loud noises • The fetus’s bones have already been developing, but they now start to ossify, beginning with the clavicles and bones in the legs. The bone marrow also develops and starts producing blood cells. Prior to this time, blood cells were produced by the liver and spleen. • Alveoli form in the lungs. These functional units of the lungs must be fully developed before an infant can breathe air after birth. • Considerable muscle development occurs. The fetus’s movements become more forceful, and the movements stimulate further development of the muscles. • The intestines develop sufficiently that small amounts of sugars can be absorbed from the amniotic fluid that is swallowed. Virtually all of the fetus’s nutrients, however, still come from the mother’s blood via the placenta. • The fetus develops a thick waxy coating called vernix. This coating protects the fetus’s skin from becoming chapped or irritated by amniotic fluid. 2ND TRIMESTER COMPLICATIONS • Gestational diabetes • An incompetent cervix, • placental abruptions • Symptoms to be aware of include bleeding and both cramping and tenderness of the uterus. WEEKS 27 TO 38 • The bones of the fetus complete their development. • Formerly wrinkled skin starts to plump out as layers of subcutaneous fat are deposited. • The fetus’s head hair grows thicker and coarser while the lanugo is shed. • In preparation for breathing after birth, the fetus will repeatedly mimic breathing by moving the diaphragm. By about week 32, the lungs are likely to be fully developed so the fetus can breathe on its own • The fetus can not only hear and feel touch, but its eyes can now detect light. In fact, the pupils can constrict and dilate in response to light. • During this phase, the fetus sleeps much of the time. Its brain, however, is continuously active. 3RD TRIMESTER COMPLICATIONS • Preeclampsia may develop at any time during the second half of pregnancy, there is a higher risk of it developing during this stage. • Gestational diabetes • An incompetent cervix, • Preterm labor • Preterm rupture of the membranes • Placenta previa • Intrauterine growth restriction • Post-term pregnancy • malpresentation GENETIC AND ENVIRONMENTAL RISKS TO EMBRYONIC DEVELOPMENT • Alcohol consumption: Exposure of the embryo to alcohol from the mother’s blood can cause fetal alcohol spectrum disorder. Children born with this disorder may have cognitive deficits, developmental delays, behavioral issues, and distinctive facial features. • Radiation from diagnostic X-rays or radiation therapy in the mother: Radiation may damage DNA and cause mutations in embryonic germ cells. When mutations occur at such an early stage of development, they are passed on to daughter cells in many tissues and organs, which is likely to have severe impacts on the offspring. • Nutritional deficiencies: A maternal diet lacking certain nutrients may cause birth defects. The birth defect called spina bifida is caused by a lack of folate when the nervous system is first forming, which happens early in the embryonic stage. In this disorder, the neural tube does not close completely and may lead to paralysis below the affected region of the spinal cord. GENETIC AND ENVIRONMENTAL RISKS TO FETAL DEVELOPMENT • Multiple environmental risk factors, such as exposure to endocrine disrupting chemicals (EDCs), smoking, air pollution, can have a range of impacts on the health of a growing fetus . EDCs have the potential to interfere with endogenous hormone action. Several studies have suggested adverse endocrine disruptive effects of EDCs on the fetus, such as miscarriage, low birth weight, hypospadias, cryptorchidism, and other birth defects • Maternal smoking during pregnancy and secondhand smoking exposure can result in placental problems (previa and/or abruption), miscarriage, stillbirth, premature birth, and FGR • Teratogens can begin affecting the developing embryo as early as 10 to 14 days after conception. During embryonic development, there are periods when the developing organ systems show more sensitivity to teratogens. Specifically, if exposure to a teratogen occurs during the first 3.5 to 4.5 weeks of gestation, a neural tube defect, such as spina bifida or anencephaly, may result.
