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Stages of Prenatal Development
Prenatal Development Ultrasound Picture
Early Signs of Pregnancy
For most women, the first symptom of pregnancy is the lack of a regular menstrual period. Some women may experience swollen, tender breasts and feelings of nausea at the time of the missed period, while others will have almost no symptoms at all. What are the earliest signs of pregnancy?
- A missed menstrual period.
- Spotting near the time of the expected menstrual period (implantation bleeding).
- Swollen or tender breasts.
- Increased trips to the bathroom (frequent urination).
- Increased pigmentation of the skin (the "mask of pregnancy," or melasma).
- Nausea, which may lead to vomiting.
Pregnancy Tests and Detection Methods
The most common pregnancy test is an over-the-counter urine dip-stick. These test kits can be purchased at any pharmacy or big-box store. The most sensitive tests can identify a pregnancy 4a few days before your period is due, though it is usually best to wait for a missed period before testing for the most accurate results. Home pregnancy tests are approximately 97% accurate at detecting a pregnancy.
A more accurate test is a blood pregnancy test, ordered by a physician. This test measures the exact level of Human Chorionic Gonadotropin (hCG), a hormone secreted by the developing embryo. The blood test may be very useful for tracking increasing levels of pregnancy hormone, as a healthy pregnancy will result in increasing hCG over the first gestational months.
Another test that is not widely available determines whether a hormone called "Early Pregnancy Factor" (EPF) is present. EPF is present by 24-48 hours after fertilization. This is a blood test and cannot be routinely obtained.
A Positive Pregnancy Test
Fertilization and Early Pregnancy
Fertilization of the Egg
The fertilization and implantation process of the early embryo is the most dangerous period of time for a pregnancy. According to a study in the New England Journal of Medicine, up to 80% of fertilized eggs may not implant and will fail to develop into a pregnancy1. Once implanted, a further 31% of embryos will miscarry. The primary cause of miscarriage in the earliest stages is due to genetic mutations and chromosomal imbalances that prevent the embryo from developing. Others estimate that approximately 2/3 of all fertilization attempts do not result in pregnancy. Generally, this is the cause behind a "chemical" pregnancy, or a very early miscarriage. A pregnancy test may result in a positive result, and the woman may have a slightly late period as the embryo fails to develop or implant.
What happens when an egg is fertilized and everything goes right? The human sperm contains 23 individual chromosomes, and the human egg contains 23 individual chromosomes. When the first sperm cell penetrates the the egg, a phenomenon known as the cortical reaction occurs. The egg's membrane experiences a change in electrical charge, which immediately repulses any other sperm away from the egg. A release of calcium ions into the egg's interior causes cortical granules to bind with the egg's membrane: enzymes released by the granules disintegrate molecular sperm binding spots on the egg's exterior. An "envelope" (called the vitelline envelope) forms around the egg. These combined actions prevent more than one sperm from penetrating the egg.
Once inside the egg, the sperm releases its genetic material, which combines with the egg's chromosomes to form 23 pairs of chromosomes. Gender, hair color, eye color, and all other genetic characteristics are determined at this point in time. Once this has occurred, cell division begins.
1Wilcox AJ, Weinberg CR, O'Connor JF, Baird DD, Schlatterer JP, Canfield RE, Armstrong EG, Nisula BC: Incidence of early loss of pregnancy. New Engl J Med 319:189-94, 1988
Embryonic Cell Division and ImplantationClick thumbnail to view full-size
Implantation and Development of the Early Embryo
The fertilized egg is called a zygote, and will begin to divide. By the third day after fertilization, the embryo is shaped like a sphere and has 12 to 16 cells. This early cell mass is called a morula. As the morula moves down the Fallopian tube, it is protected by an outer covering called the zona pellucida. This covering prevents the embryo from implanting too early in the Fallopian tube.
Approximately four days after fertilization, the embryo enters the uterus and the zona pellucida disintegrates, allowing the cells to embed into the uterine lining (the endometrium). The emergence from the zona pellucida is sometimes called "hatching." The cells have developed into a blastocyst, and have an inner cell mass (which will become a baby) and another set of cells that will develop into the placenta. The blastocyst begins attaching to the uterine wall, and implantation takes place from 6-12 days after fertilization. By the twelfth day after fertilization, the embryo has implanted and consists of several hundred cells.
Early Embryonic Development
The First Trimester: Weeks 1-5
The first four weeks of pregnancy often pass by without the mother knowing she's pregnant. Pregnancy weeks are counted from the date of the mother's last menstrual cycle rather than from the date of fertilization. This is because the exact date of fertilization cannot be determined: the gestational age of the embryo will be two weeks less than the "week" of the pregnancy.
Weeks 1-2 of a pregnancy actually happen before fertilization has occurred. Approximately 2 weeks into a woman's cycle, ovulation will occur. Fertilization happens at this time point. Weeks 3-4 consist of the fertilized egg's journey to the uterus and subsequent implantation. Some very early pregnancy symptoms may be felt toward the end of this time period.
Week 5 is when most women will notice a delay in the onset of their menstrual cycle and will purchase a pregnancy test - the embryo is approximately 3 weeks old at this point in time. "Morning sickness," tender breasts, lack of a menstrual period, and feeling of tiredness may be evident. The embryo consists of three layers: an ectoderm that will develop into skin and hair, the mesoderm that will develop into the heart, bones, and kidneys, and the endoderm that will develop into the lining of the lungs, intestines, and the bladder. Blood cells are produced by a yolk sac. By day 18 after fertilization, the embryo's heart appears.
The First Trimester: In the Womb
The First Trimester: Weeks 6-12
This period of development is extremely fragile for a young embryo. Chemicals, toxins, and other environmental dangers are the most likely to do harm during this developmental time period.
Week 6 of the pregnancy (week 4 gestational age) is a very busy one. The embryo's heart is beating, the spinal column is developing, and the embryo flattens and folds to incorporate the yolk sac (which begins to form the digestive tract). Early limb buds appear on the embryo, which becomes entirely enveloped in the amniotic sac. Amniotic fluid is now present to protect the embryo.
Week 7 (5 weeks after fertilization) is just as busy as week 6. The embryo appears similar to a tadpole, and the head constitutes 1/3 of the total body length. The cerebellum, responsible for muscle control and coordination, appears this week and the eyes begin to develop. At the end of this week, the embryo's liver begins to form its own blood cells.
Week 8 (6 weeks after fertilization) results in the development of cartilage, the cerebral cortex (responsible for higher thought processes), and the inner ear. The embryo begins to move at this stage of development, though the mother won't feel any movement for quite some time. Fingers begin to form on the hands,and the pancreas begins to function.
Week 9 (7 weeks gestational age) starts off with the development of brain waves that can be recorded by EEG, and the heart develops four chambers. The embryo begins to develop muscles and the bones in the jaw and collar bone begin to harden.
The tenth week of pregnancy (8 weeks post fertilization) results in the development of the kidneys, which now excrete urine into the amniotic fluid. The fingers are well formed and the brain is over 40% of the baby's weight. The eyelids grow and fuse together.
Weeks 11-12 of pregnancy (9-10 weeks after fertilization) are when the fetus experiences a major growth spurt - the baby's weight increases by over 70%! The baby may suck his or her thumb, the bones are hardening, and fingerprints develop.
Fetal Movement Poll
When did you first feel fetal movement?
The Second Trimester: Weeks 13-28
By the second trimester, all of the major organs and body systems are in place. The baby will continue to grow and the organs will continue to develop over this period of time.
In the 3rd and 4th month, the baby's skin will thicken, the plugs in the nostrils disappear, and the first identifiable signs of gender begin to appear around 14 weeks. 14 weeks is also the earliest time a mother may feel movement from her baby - the little one is now big enough to make an impact with his or her twists and turns!
In the 4th and 5th month, the baby begins to practice breathing motions. A waxy, white substance called vernix begins to cover the baby's skin, to protect it from the watery environment of the womb. Some of this whitish layer will still be evident once the baby is born. By 20 weeks of pregnancy, the hearing organ of the cochlea has reached its adult size. The baby can now hear.
In the 6th and 7th month, the baby's gastrointestinal tract begins to function. The eyelids open at 24 weeks of gestation, and the baby is able to blink. The "startle reflex" in newborns is now apparent, and the baby will startle to loud noises in utero. It is possible for the baby to survive outside the womb at this stage, though intensive care will be required in a specialized neonatal unit. The lungs begin to produce small amounts of surfactant (a chemical necessary for breathing air) at approximately 24 weeks, but the lungs are far from developed. An infant born at this stage of pregnancy would require surfactant to keep the lungs open after birth.
The Last Trimester: Lung Development and Fetal Surgery
The Third Trimester: Weeks 29-40
The last trimester is when the baby really begins to put on weight, and the lungs finish their development. The baby practices breathing movements for at least 1/3 of his or her day, and the lungs begin to develop alveoli at about 32 weeks. Interestingly, the lungs continue to develop alveoli (air sacs) until the age of 8 years!
The final months leading up to birth are a time for the baby's lungs to finish developing and for the baby to put on weight. The brain becomes twice as heavy during the last 2 months of gestation.
The initiation of labor is a complex event, but is triggered in part by the release of sufficient quantities of surfactant from the full-term baby's lungs. The protein Surfactant Protein A (SP-A) is released into the amniotic fluid as one of the first precursors to labor. Other proteins and hormones are involved, as the relative amount of progesterone decreases and the uterus is stretched by the ever-growing baby.
Breathing in a New Way
Birth: The Full Term Baby
A full term infant is well developed, but must transition from taking oxygen through the umbilical cord to breathing air. The birth process helps to force fluid out of the lungs (babies born through a Caesarean section may require suctioning). Taking the first breath is a complicated process.
When the infant takes its first breath of life, the circulation system changes in a dramatic way. One hole in the heart closes forever, changing the way blood flows through the body. A blood vessel also seals off (the ductus arteriosus). This shunts the blood to the lungs, which had been previously bypassed. The ductus arteriosus closes and becomes a ligament by 5 days after birth. The foramen ovale closes within an hour of birth and remains sealed for life.