The Common Vein Copyright 2010
Introduction
Pregnancy is a physiologic process caused by the fusion of two gametes after sexual intercourse. This fusion, called fertilization, takes place in the fallopian tubes, and the result is a zygote, a diploid cell that will grow into an embryo.
The function of every biological unit in the body is based on its ability to receive a product, process it, and deliver the new product efficiently and effectively to where it is used and needed. The primary function of the uterus is to receive and hold the embryo, support its growth and development throughout gestation and ultimately deliver a well-formed, viable neonate.
Receiving: Fertilization and Implantation
Fertilization begins with the ovum being released from the ovary. At the time of ovulation, the ovum is fixed within the process of meiosis at the metaphase II stage. It is surrounded by the zona pellucida, a layer of mucopolysaccharide, and the cumulus oophorus, a layer of granulosa cells from the ovarian follicle. The ovum is swept up by the fimbriae of the fallopian tubes. If sexual intercourse occurs around the time of ovulation, sperm are released and move from the vagina through the cervical mucus into the uterus and fallopian tubes. The sperm move towards the ovum through a chemical attraction called chemotaxis. In the uterus, the sperm undergo capacitation, a reaction that makes the sperm membrane more permeable and better able to fuse with the oocyte membrane. When the sperm reach the ovum in the fallopian tube, they undergo the acrosome reaction, releasing enzymes from the sperm head that allow it to pass through the zona pellucida. The sperm membrane fuses with the oocyte membrane and the sperm head containing the gamete?s genetic material passes into the oocyte?s cytoplasm. The oocyte completes its second meiotic division, and the result is a diploid zygote containing two pronuclei, membrane bound structures each containing a maternal or paternal set of chromosomes. The membranes of these pronuclei dissolve and the chromosomes from the sperm and oocyte pair in preparation for the zygote?s first mitotic division. This cell will divide multiple times as the embryo travels through the fallopian tube into the uterine cavity. This process usually takes 3-4 days after fertilization.
As the embryo travels into the uterus, its cells grow, divide and begin to differentiate. The first cells to differentiate are the trophoblastic cells, cells that will become the placenta. Enzymes from the endometrial glands digest the remaining zona pellucida in order to allow these cells to attach to and invade the endometrium. These cells proliferate within the endometrium, invading the small capillaries to form the vascular sinuses of the early placenta. Through these venous sinuses, the cells of the growing placenta secrete progesterone and HCG, which allows detection of the pregnancy with laboratory testing.
The uterus undergoes multiple structural and functional changes throughout pregnancy. Early in pregnancy, the endometrium, in the secretory phase of the menstrual cycle, provides a suitable environment for implantation of the embryo.
Some disorders resulting from aberrant fertilization and implantation include infertility, miscarriage and ectopic pregnancy. Diagnosis is made based on clinical history, physical exam and imaging with ultrasound. Treatment of infertility can include induction of ovulation with hormones, intrauterine insemination or in vitro fertilization. Miscarriage may require no treatment, but dilation and curettage is often used to ensure full removal of the products of conception. Ectopic pregnancy may be treated medically, with methotrexate, or surgically with removal of the ectopic gestation.
Processing: Growth and Nourishment
After implantation the embryo grows and the cells continue to differentiate. Around the third week of gestation, the cells differentiate into ectoderm, endoderm and mesoderm. The endoderm will develop into the gastrointestinal and respiratory tracts. The mesoderm will differentiate into the cardiovascular, genitourinary and musculoskeletal systems. The ectoderm will form the nervous system, skin and sensory organs. Organogenesis occurs between the 4th and 7th week of gestation. After the 8th week of gestation, the embryo is called a fetus. This fetus will grow and develop within the uterus over 40 weeks of pregnancy. The placenta will grow and invade the arterioles of the endometrium to connect maternal and fetal circulation.
Throughout gestation the uterus grows to accommodate the developing fetus. The myometrium becomes more muscular and elastic. The uterus stretches to 500 times in size, thinning the uterine wall. As the nutrition needs of the fetus increase, the uterine blood supply hypertrophies to increase blood flow in the maternal-fetal circulation.
There are many congenital disorders resulting from aberrant growth and development of the fetus. Some disorders resulting from uterine dysfunction during gestation include spontaneous abortion and cervical insufficiency. Diagnosis of these conditions is made by clinical history and exam combined with ultrasound imaging during pregnancy. Often no treatment is necessary for spontaneous abortion. Women with recurrent abortions should be tested for uterine structural abnormalities as well as gynecologic infections. Cervical insufficiency may be treated expectantly with steroids to promote fetal lung maturity and bed rest. Cervical cerclage may also be used to treat cervical insufficiency.
Delivery: Labor and Childbirth
Labor is uterine contractions leading to cervical changes caused by contraction of the myometrium in response to the hormone oxytocin. The result is delivery of the fetus and placenta. Between the 36th and 40th week of gestation, the irregular contractions of the myometrium become rhythmic. The membranes surrounding the fetus and amniotic fluid rupture. The cervix effaces and dilates up to 10 cm in diameter. As the cervix dilates, the plug of cervical mucus is expelled resulting in the bloody show. The fetus descends into the pelvis and rotates as the strengthening rhythmic contractions of the uterus propel the fetus through the birth canal. After delivery of the neonate, uterine contractions continue delivering the placenta.
During labor and delivery the uterus undergoes several important structural and functional changes. The cervix dilates and effaces allowing the fetus to pass. The myometrium contracts rhythmically to delivery the neonate and placenta. After childbirth, the uterine vasculature constricts and the myometrium contracts to stop postpartum bleeding.
Pregnancy complications that occur around or during labor and delivery include placental bleeding, uterine rupture, premature rupture of membranes or labor, cephalopelvic disproportion and malpresentation. Diagnosis of these conditions is made based on clinical history, exam, and imaging with ultrasound. Treatment of placental bleeding (placenta previa or placental abruption) depends on the severity of bleeding. Premature rupture of membranes is often treated with antibiotics, and sometimes induction of labor. Preterm labor is treated with tocolysis and steroids to promote fetal lung maturity. Uterine rupture is a medical emergency treated with immediate laparotomy and cesarean delivery. Cephalopelvic disproportion and malpresentation both often require cesarean delivery.