Pregnancy

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This article is about pregnancy in humans. For other mammals, see Pregnancy (mammals). For fish, see Pregnancy in fish.

Pregnancy, also known as gestation, is the time during which one or more offspring develops inside a woman. A multiple pregnancy involves more than one offspring, such as with twins.Pregnancy usually occurs by sexual intercourse, but can also occur through assisted reproductive technology procedures. A pregnancy may end in a live birth, a spontaneous miscarriage, an induced abortion, or a stillbirth. Childbirth typically occurs around 40 weeks from the start of the last menstrual period (LMP).This is just over nine months (gestational age)—where each month averages 31 days.] When using fertilization age it is about 38 weeks. An embryo is the developing offspring during the first eight weeks following fertilization, (ten weeks' gestational age) after which, the term fetus is used until birth. Signs and symptoms of early pregnancy may include missed periods, tender breasts, morning sickness (nausea and vomiting), hunger, and frequent urination.Pregnancy may be confirmed with a pregnancy test.

Pregnancy is divided into three trimesters of approximately 3 months each. The first trimester includes conception, which is when the sperm fertilizes the egg. The fertilized egg then travels down the Fallopian tube and attaches to the inside of the uterus, where it begins to form the embryo and placenta. During the first trimester, the possibility of miscarriage (natural death of embryo or fetus) is at its highest. Around the middle of the second trimester, movement of the fetus may be felt. At 28 weeks, more than 90% of babies can survive outside of the uterus if provided with high-quality medical care, though babies born at this time will likely experience serious health complications such as heart and respiratory problems and long-term intellectual and developmental disabilities.

Prenatal care improves pregnancy outcomes. Prenatal care may include taking extra folic acid, avoiding drugs, tobacco smoking, and alcohol, taking regular exercise, having blood tests, and regular physical examinations. Complications of pregnancy may include disorders of high blood pressure, gestational diabetes, iron-deficiency anemia, and severe nausea and vomiting. In the ideal childbirth labor begins on its own when a woman is "at term". Babies born before 37 weeks are "preterm" and at higher risk of health problems such as cerebral palsy. Babies born between weeks 37 and 39 are considered "early term" while those born between weeks 39 and 41 are considered "full term". Babies born between weeks 41 and 42 weeks are considered "late term" while after 42 weeks they are considered "post term". Delivery before 39 weeks by labor induction or caesarean section is not recommended unless required for other medical reasons.

About 213 million pregnancies occurred in 2012, of which, 190 million (89%) were in the developing world and 23 million (11%) were in the developed world. The number of pregnancies in women aged between 15 and 44 is 133 per 1,000 women. About 10% to 15% of recognized pregnancies end in miscarriage. In 2016, complications of pregnancy resulted in 230,600 maternal deaths, down from 377,000 deaths in 1990. Common causes include bleeding, infections, hypertensive diseases of pregnancy, obstructed labor, miscarriage, abortion, or ectopic pregnancy. Globally, 44% of pregnancies are unplanned. Over half (56%) of unplanned pregnancies are aborted. Among unintended pregnancies in the United States, 60% of the women used birth control to some extent during the month pregnancy occurred.

Terminology

Associated terms for pregnancy are gravid and parous. Gravidus and gravid come from the Latin word meaning "heavy" and a pregnant female is sometimes referred to as a gravida. Gravidity refers to the number of times that a female has been pregnant. Similarly, the term parity is used for the number of times that a female carries a pregnancy to a viable stage. Twins and other multiple births are counted as one pregnancy and birth. A woman who has never been pregnant is referred to as a nulligravida. A woman who is (or has been only) pregnant for the first time is referred to as a primigravida, and a woman in subsequent pregnancies as a multigravida or as multiparous. Therefore, during a second pregnancy a woman would be described as gravida 2, para 1 and upon live delivery as gravida 2, para 2. In-progress pregnancies, abortions, miscarriages and/or stillbirths account for parity values being less than the gravida number. In the case of a multiple birth the gravida number and parity value are increased by one only. Women who have never carried a pregnancy more than 20 weeks are referred to as nulliparous.

A pregnancy is considered term at 37 weeks of gestation. It is preterm if less than 37 weeks and postterm at or beyond 42 weeks of gestation. American College of Obstetricians and Gynecologists have recommended further division with early term 37 weeks up to 39 weeks, full term 39 weeks up to 41 weeks, and late term 41 weeks up to 42 weeks. The terms preterm and postterm have largely replaced earlier terms of premature and postmature. Preterm and postterm are defined above, whereas premature and postmature have historical meaning and relate more to the infant's size and state of development rather than to the stage of pregnancy.

Signs and symptoms

Main article: Signs and symptoms of pregnancy Further information: Complications of pregnancy

Melasma: pigment changes to the face due to pregnancy

In the later part of pregnancy the uterus takes up much of the abdomen The usual signs and symptoms of pregnancy do not significantly interfere with activities of daily living or pose a health-threat to the mother or baby. However, pregnancy complications can cause other more severe symptoms, such as those associated with anemia.

Common signs and symptoms of pregnancy include:

Tiredness Morning sickness Constipation Pelvic girdle pain Back pain Braxton Hicks contractions. Occasional, irregular, and often painless contractions that occur several times per day. Peripheral edema swelling of the lower limbs. Common complaint in advancing pregnancy. Can be caused by inferior vena cava syndrome resulting from compression of the inferior vena cava and pelvic veins by the uterus leading to increased hydrostatic pressure in lower extremities. Low blood pressure often caused by compression of both the inferior vena cava and the abdominal aorta (aortocaval compression syndrome). Increased urinary frequency. A common complaint, caused by increased intravascular volume, elevated glomerular filtration rate, and compression of the bladder by the expanding uterus. Urinary tract infection Varicose veins. Common complaint caused by relaxation of the venous smooth muscle and increased intravascular pressure. Hemorrhoids (piles). Swollen veins at or inside the anal area. Caused by impaired venous return, straining associated with constipation, or increased intra-abdominal pressure in later pregnancy. Regurgitation, heartburn, and nausea. Stretch marks Breast tenderness is common during the first trimester, and is more common in women who are pregnant at a young age. Melasma, also known as the mask of pregnancy, is a discoloration, most often of the face. It usually begins to fade several months after giving birth.

Estimation of due date

Main article: Estimated date of delivery

Due date estimation basically follows two steps:

Determination of which time point is to be used as origin for gestational age, as described in the section above.

Adding the estimated gestational age at childbirth to the above time point. Childbirth on average occurs at a gestational age of 280 days (40 weeks), which is therefore often used as a standard estimation for individual pregnancies. However, alternative durations as well as more individualized methods have also been suggested.

Naegele's rule is a standard way of calculating the due date for a pregnancy when assuming a gestational age of 280 days at childbirth. The rule estimates the expected date of delivery (EDD) by adding a year, subtracting three months, and adding seven days to the origin of gestational age. Alternatively there are mobile apps, which essentially always give consistent estimations compared to each other and correct for leap year, while pregnancy wheels made of paper can differ from each other by 7 days and generally do not correct for leap year.

Furthermore, actual childbirth has only a certain probability of occurring within the limits of the estimated due date. A study of singleton live births came to the result that childbirth has a standard deviation of 14 days when gestational age is estimated by first trimester ultrasound, and 16 days when estimated directly by last menstrual period.

Maternal changes

Main article: Maternal physiological changes in pregnancy

During pregnancy, a woman undergoes many physiological changes, which are entirely normal, including behavioral, cardiovascular, hematologic, metabolic, renal, and respiratory changes. Increases in blood sugar, breathing, and cardiac output are all required. Levels of progesterone and estrogens rise continually throughout pregnancy, suppressing the hypothalamic axis and therefore also the menstrual cycle. A full-term pregnancy at an early age reduces the risk of breast, ovarian and endometrial cancer and the risk declines further with each additional full-term pregnancy.

The fetus is genetically different from its mother, and can be viewed as an unusually successful allograft. The main reason for this success is increased immune tolerance during pregnancy. Immune tolerance is the concept that the body is able to not mount an immune system response against certain triggers.

During the first trimester, minute ventilation increases by 40%. The womb will grow to the size of a lemon by eight weeks. Many symptoms and discomforts of pregnancy like nausea and tender breasts appear in the first trimester.

By the end of the second trimester, the expanding uterus has created a visible "baby bump". Although the breasts have been developing internally since the beginning of the pregnancy, most of the visible changes appear after this point. During the second trimester, most women feel more energized, and begin to put on weight as the symptoms of morning sickness subside and eventually fade away. The uterus, the muscular organ that holds the developing fetus, can expand up to 20 times its normal size during pregnancy.

Braxton Hicks contractions are sporadic uterine contractions that may start around six weeks into a pregnancy however, they are usually not felt until the second or third trimester.

Final weight gain takes place during the third trimester, which is the most weight gain throughout the pregnancy. The woman's abdomen will transform in shape as it drops due to the fetus turning in a downward position ready for birth. During the second trimester, the woman's abdomen would have been upright, whereas in the third trimester it will drop down low. The fetus moves regularly, and is felt by the woman. Fetal movement can become strong and be disruptive to the woman. The woman's navel will sometimes become convex, "popping" out, due to the expanding abdomen. l Head engagement, also called "lightening" or "dropping" occurs as the fetal head descends into a cephalic presentation. While it relieves pressure on the upper abdomen and gives a renewed ease in breathing, it also severely reduces bladder capacity resulting in a need to void more frequently, and increases pressure on the pelvic floor and the rectum. It is not possible to predict when lightening occurs. In a first pregnancy it may happen a few weeks before the due date, though it may happen later or even not until labor begins, as is typical with subsequent pregnancies.

It is also during the third trimester that maternal activity and sleep positions may affect fetal development due to restricted blood flow. For instance, the enlarged uterus may impede blood flow by compressing the vena cava when lying flat, which is relieved by lying on the left side.

Childbirth

Main article: Childbirth

Further information: Pain management during childbirth

Childbirth, referred to as labor and delivery in the medical field, is the process whereby an infant is born.

A woman is considered to be in labour when she begins experiencing regular uterine contractions, accompanied by changes of her cervix—primarily effacement and dilation. While childbirth is widely experienced as painful, some women do report painless labours, while others find that concentrating on the birth helps to quicken labour and lessen the sensations. Most births are successful vaginal births, but sometimes complications arise and a woman may undergo a cesarean section.

During the time immediately after birth, both the mother and the baby are hormonally cued to bond, the mother through the release of oxytocin, a hormone also released during breastfeeding. Studies show that skin-to-skin contact between a mother and her newborn immediately after birth is beneficial for both the mother and baby. A review done by the World Health Organization found that skin-to-skin contact between mothers and babies after birth reduces crying, improves mother–infant interaction, and helps mothers to breastfeed successfully. They recommend that neonates be allowed to bond with the mother during their first two hours after birth, the period that they tend to be more alert than in the following hours of early life.