Stages of fetal development over 38 weeks, from embryo to full-term.
Credit: Dragana Gerasimoski | Shutterstock
Having a baby is a joyous and wonderful experience, but many people don't understand the stages of pregnancy or know what to expect with each one. Human pregnancy generally lasts for 40 weeks and is divided into three stages, or trimesters. Let's start with understanding what happens at conception.
Conception and implantation
Every month inside a woman's ovaries, eggs begin to grow in follicles, which are small fluid sacs. During ovulation, which generally happens two weeks before a period, one of the eggs will erupt from the follicle and the egg is released. The follicle then develops into the corpus luteum, which releases a hormone that thickens the uterine lining in preparation for fertilization.
The egg travels into the fallopian tube, where it sits for about a day, waiting for a sperm to fertilize it. If sperm is not present for fertilization, the egg moves through the uterus and disintegrates, the thick uterine lining is shed, and the period begins. But if a sperm is present, the egg fertilizes and changes its properties so that no other sperm can enter. At this point, the egg becomes an embryo, and its genes and sex are set — two X chromosomes denote a girl and an X chromosome and a Y chromosome denote a boy.
After conception, implantation begins. The embryo stays in the fallopian tube for anywhere from three to four days, and as it moves toward the uterus, the cells begin to divide and develop. Once it reaches the uterus, the embryo attaches or "implants" to the lining of the uterus. This can cause spotting or light bleeding for a couple of days for the woman. At this time, the uterine lining thickens, and the cervix is sealed by a mucus plug, which will stay in place until birth.
Through the first week, levels of the hormone human chorionic gonadotropin (hCG) will begin to rise, created by the cells that become the placenta. As pregnancy progresses, hCG increases and can be detected more easily, either via blood or urine tests. For home pregnancy tests, this is usually around three to four weeks into the pregnancy. At the same time, the cells start to grow as clumps, forming the baby's first nerve cells.
From week 1 to week 12, the hormonal changes begin and start to affect pretty much every organ in the body. Symptoms include tiredness, swollen breasts, upset stomach or nausea, food cravings, mood swings, and headaches. Weight gain or loss may begin at this point as well.
These symptoms will require changes to your daily routine. Consider going to bed earlier, or eating frequent and small meals. You can also talk to your doctor about suggestions for combating your symptoms — you don't want to take any medication or supplements without first consulting a healthcare provider.
As the pregnancy progresses, many of these discomforts will pass. Some women may not even experience any of the negative symptoms associated with the first trimester. It's important to remember that every pregnancy is different.
Within the first trimester, there are a number of developments occurring with the embryo. At four weeks, the brain and spinal cord have begun to form, as well as the heart and limb buds. The embryo is now about 1/25 of an inch (1 millimeter) long. By eight weeks, the major organs and body structure have begun to form, and the heartbeat is at a regular rhythm. Sex organs begin to form, and the face begins to take shape as well. After eight weeks, the embryo, now called a fetus, is nearly an inch (2.5 centimeters) long.
At 12 weeks, the fetus begins to use nerves and muscles together, like making a fist. The external sex organs are almost developed, and eyelids close to protect the eyes. At 3 inches long, the fetus weighs almost an ounce.
Weeks 13-28 are often easier on women than the first 1-12, often referred to as the "golden period" of pregnancy. The nausea and fatigue may begin to go away, but physical changes will become more apparent. The abdomen will begin to expand, and the back, abdomen or thighs may begin to ache. Stretch marks will become more apparent as the areas expand, and skin may begin to darken on the face and nipples. On the face, this is often referred to as the mask of pregnancy.
Some more alarming symptoms may include numb or tingling hands, caused by carpal tunnel syndrome. The abdomen, palms or soles of the feet may begin to itch, and the ankles, fingers and face may begin to swell. However, if the itching is combined with nausea or jaundice, the FDA's Office of Women's Health recommends contacting a doctor immediately, as this could be a sign of liver disease. Sudden or extreme swelling also warrants a call to the doctor, as this could be a sign of preeclampsia.
At 16 weeks, the fetus' skeleton begins to form, as well as skin. Four weeks later, defining hair features such as eyebrows and eyelashes begin to form, and the fetus can now hear and swallow. Before this trimester is over, the fetus, now up to 12 inches (30 cm) long and weighing about one and a half pounds (680 grams), will begin to move inside the womb. Sex organs will develop by the 24th week, and bone marrow begins to develop inside the bones.
Weeks 29-40 of pregnancy may include many of the second trimester symptoms, plus breathing problems and frequent urination. As the fetus grows, it puts more and more pressure on the mother's organs. Additional symptoms include hemorrhoids, leaking tender breasts, belly button sticking out, and false labor pains. The fetus may move lower in the abdomen, and also turn into a head-down position for birth. The cervix begins the effacing process to become thinner and softer, helping the birth canal open during the birthing process. As the due date nears, the doctor should be checking this progress with vaginal exams.
At 32 weeks, the fetus' bones are finally fully formed, but are still soft. You may notice some forceful jabs and kicks. The fetus' senses will start to heighten, including opening and closing eyes. At this point, he or she will gain more weight per week and will be around 16 inches (40 cm) long and roughly 4 pounds (1.8 kilograms).
By the end of the 37th week, the fetus' organs can function on their own, and he or she is considered full-term. Most fetuses at this point will fall between 19 and 21 inches (48 to 54 cm) and weigh between 6 and 9 pounds (2.7 to 4 kg), but a healthy baby can come in many different sizes. Infants delivered before 37 weeks are considered premature, and may have difficulties breathing or digesting. Development and growth may also be stunted.