Misplaced Pages

This is an old revision of this page, as edited by 24.126.254.36 (talk) at 04:16, 29 June 2005 (→External links). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

Human pregnancy refers to the process by which a human female carries a live offspring from conception until childbirth. The medical term for a pregnant woman is "gravida," just as the medical term for the unborn human is embryo and then fetus.

Pregnancy takes approximately 40 weeks between the time of the last menstrual cycle and birth (38 weeks from fertilization). It is divided into three trimesters. The first trimester carries the highest risk of miscarriage, the natural death of an embryo or fetus, known medically as a spontaneous abortion. It is often the result of health problems of the fetus, the mother, or damage caused after conception.

Fertilization

Pregnancy is the result of the fusion of two gametes (spermatocyte and oocyte) into a zygote. This process is termed fertilization or conception. This process occurs in the Fallopian tubes or in the uterus. The zygote then becomes embedded into the endometrium (lining of the uterus) where it forms a placenta to obtain nutrients for growth.

Duration

Traditionally (according to Naegele's Rule), a human pregnancy is considered to last approximately 40 weeks (280 days) from the last menstrual period (LMP), or 38 weeks (266 days) from the date of conception. However, a pregnancy is considered to have reached term between 38 and 42 weeks. Babies born before the 37 week mark are considered premature, while babies born after the 42 week mark are considered postmature.

However, the average length of pregnancy depends on ethnic background of the mother (Caucasian women are more likely to have a longer pregnancy than other women) and if it is a first pregnancy (which tend to last longer than subsequent pregnancies). For example, a Caucasian woman's first pregnancy lasts an average 274 days from conception (288 days from the last menstrual period).

An accurate date of conception is important, because it is used in calculating the results of various prenatal tests (for example, in the triple screen test). A decision may be made to induce labour if a baby is perceived to be overdue. Due dates are only a rough estimate, and the process of accurately dating a pregnancy is complicated by the fact that not all women have 28 day menstrual cycles, or ovulate on the 14th day following their last menstrual period. Approximately 3.6% of all mothers deliver on the due date predicted by LMP, and only 4.7% give birth on the day predicted by ultrasound.

Childbirth

Childbirth is the process in which the baby is born. It may come about naturally or be induced for medical reasons.

Postnatal

For topics following on from a successful pregnancy and birth, see:

• Breastfeeding
• Child development

Effects on the mother

By trimester

First trimester: Hormonal changes are the strongest, causing symptoms like morning sickness, fatigue, mood swings and food cravings. Pregnancy is usually suspected two to three weeks after conception.

Second trimester: Morning sickness usually disappears. Abdomen begins to bulge, starting the "obvious phase" of pregnancy. Hyperpigmentation, including linea nigra may appear.

Third trimester: Mother may experience backaches due to increased strain. The spine is arched in order to counteract weight. The mother may also suffer mild urinary incontinence due to pressure on the bladder by the pregnant uterus, as well as heartburn (due to compression of the stomach).

Overall

During pregnancy, the woman undergoes many physiological changes, be they cardiovascular, renal, hematologic, metabolic or respiratory, changes that become very important in the event of complications.

The woman is the sole provider of nourishment for the embryo and later, fetus, and so her plasma and blood volume increase by 40-50% to accommodate the changes. This results in overall vasodilation, increased heart rate (15 beat/min), stroke volume, and cardiac output, ultimately increasing cardiac capacity by 70-80mL. Diastolic blood pressure consequently decreases between 12-26 weeks, and increases again to prepregnancy levels by 36 weeks. If the blood pressure remains abnormal beyond 36 weeks, the woman should be investigated for pre-eclampsia, a condition that precedes eclampsia.

Hematologically, the increase in plasma volume causes an increase in red blood cell mass. The pregnant woman also becomes hypercoagulable due to increased liver production of coagulation factors.

Pregnant women often have the sensation of shortness of breath. However, pregnant women oxygenate well, and maintain a total lung capacity comparable to non-pregnant women, with an increase in tidal volume, but a decrease in residual volume. Pregnant women breathe faster, and so have compensated respiratory alkalosis.

The increase in plasma volume results in increased levels of adrenocorticotropic hormone (ACTH), antidiuretic hormone (ADH), aldosterone, and cortisol in the kidneys, ultimately increasing the glomerular filtration rate (GFR) by 50%, a change that subsides around 20 weeks postpartum. Pregnant women may also show more proteinuria (300mg/24 hrs) than non-pregnant women (150mg/24 hrs). While this is normal, if the number rises beyond the 300mg/24 hours threshold, this would suggest renal impairment. Fully half of pregnant women will show signs of glucosuria, which is normal. If the glucosuria is persistent, gestational diabetes should be suspected.

During pregnancy, both protein metabolism and carbohydrate metabolism are affected. One kilogram of extra protein is deposited, with half going to the fetus and placenta, and another half going to uterine contractile proteins, breast glandular tissue, plasma protein, and hemoglobin.

Carbohydrate metabolism is also altered. Pregnant women have lower fasting blood glucose, and progressive insulin resistance, mostly due to human placental lactogen (HPL), which interferes with the insulin use of the body, and peaks at 24-28 weeks.

Edema, or swelling, of the feet is common during pregnancy. For the sake of comfort, many pregnant women wear larger shoes or go without. This may have something to do with the origin of the phrase "barefoot and pregnant."

Complications

Several complications can arise throughout pregnancy. In the first trimester, the two major potential problems are miscarriage and ectopic pregnancy. Another complication in the first trimester is vaginal bleeding, which occurs in nearly 25% of clinical pregnancies in the first 20 weeks. It can be the result of a miscarriage, molar pregnancy or gestational choriocarcinoma. If an expectant mother experiences bleeding in pregnancy and is Rh factor negative, she should be given an Rh immunoglobulin such as Rhogam, regardless of whether or not the bleeding proceeds to miscarriage.

Later in pregnancy, complications include gestational trophoblastic disease (GTD) and gestational choriocarcinoma.

Other complications include:

• Gestational diabetes
• Pregnancy-induced hypertension, pre-eclampsia, HELLP syndrome and eclampsia
• Morning sickness and hyperemesis gravidarum
• Pica (disorder)
• Breech birth

Imaging, monitoring and care

In present society, medical science has developed a number of procedures to monitor pregnancy.

Detection

The early stages of pregnancy is often discovered by using a pregnancy test. In the post-implantation phase the blastocyst secretes a hormone called human chorionic gonadotropin which in turn, stimulates the corpus luteum in the mother's ovary to continue producing progesterone. This acts to maintain the lining of the uterus so that the embryo will continue to be nourished. The glands in the lining of the uterus will swell in response to the blastocyst, and capillaries will be stimulated to grow in that region. This allows the blastocyst to receive vital nutrients from the mother. Pregnancy tests detect the presence of human chorionic gonadotropin.

Antenatal record

On the first visit to her gynecologist or midwife, the pregnant woman is asked to carry out the antenatal record, which constitutes a medical history and physical examination. Lab tests are also carried out, including the complete blood count (CBC), which tests for various blood values, including hemoglobin to determine if a woman is anemic, rubella vaccine, HIV test, pap smear, and a urine test.

On subsequent visits, the gestational age (GA) is rechecked with each visit. Symphysis-fundal height (SFH; in cm) should equal gestational age after 20 weeks of gestation, and the fetal growth should be plotted on a curve during the antenatal visits. The fetus is palpated by the midwife or obstetrician using Leopold maneuver to determine the position of the baby. Blood pressure should also be monitored, and may be up to 140/90 in normal pregnancies. High blood pressure indicates hypertension and possibly pre-eclampsia, if severe swelling (edema) and spilled protein in the urine are also present.

Fetal screening is also used to help assess the viability of the fetus, as well as congenital problems. Genetic counseling is often offered for families who may be at an increased risk to have a child with a genetic condition. Amniocentesis at around the 20th week is usually done for women 35 or older to check for Down's Syndrome and other chromosome abnormalities in the fetus. Even earlier than amniocentesis is performed, the mother may undergo the triple test, nuchal screening, nasal bone, alpha-fetoprotein screening and Chorionic villus sampling, also to check for disorders such as Down Syndrome. The amniocentesis is a prenatal genetic screening of the fetus, which involves inserting a needle through the mother's abdominal wall and uterine wall, to extract fetal DNA from the amniotic fluid. There is a risk of miscarriage and fetal injury witht he amniocentesis since it involves penetrating the uterus with the baby still in utero.

Imaging

Imaging is another important way to monitor a pregnancy. The mother and fetus are also usually imaged in the first trimester of pregnancy. This is done to predict problems with the mother; confirm that a pregnancy is present inside the uterus; guess the gestational age; determine the number of fetuses and placentae; evaluate for an ectopic pregnancy and first trimester bleeding; and assess for early signs of anomalies.

X-rays and computerized tomography (CT) are not used, especially in the first trimester, due to the ionizing radiation, which has teratogenic effects on the fetus. Instead, ultrasound is the imaging method of choice in the first trimester and throughout the pregnancy, since it emits no radiation, is portable, and allows for realtime imaging. Ultrasound imaging may be done at any time throughout the pregnancy, but usually happens at the 12th week (dating scan) and the 20th week (detailed scan).

A normal gestation would reveal a gestational sac, yolk sac, and fetal pole. The gestational age can be assessed by evaluating the mean gestation sac diameter (MGD) before week 6, and the crown-rump length after week 6. Multiple gestation is evaluated by the number of placentae and amniotic sacs present.

Pregnancy has different cultural aspects related to the perception of the body, the relationship with partner and to the meaning of the event.

See also

• Obstetrics
• Embryo
• Fetus
• Contraception
• Twin and Multiple birth
• Procreation
• Birth weight
• Cell division
• Oxytocin
• Teenage pregnancy
• Pregnancy discrimination
• Low birth weight paradox
• Pregnancy in science fiction
• Long-distance pregnancy
• Melasma
• Abortion

Reference

• Mittendorf R, Williams MA, Berkey CS, Cotter PF. The length of uncomplicated human gestation. Obstet Gynecol 1990;75:929-32. PMID 2342739.

External links

• The visible embryo
• Normal Pregnancy, Labor, And Delivery
• Pregnancy Exposure to Common Chemical May Harm Developing Boys health.dailynewscentral.com

• Obstetrics
• Women
• Fertility