Pregnancy and Birthing
Although pregnancy begins with implantation, the process leading to pregnancy occurs earlier as the result of the female gamete, or oocyte, merging with the male gamete, spermatozoon. In medicine this process is referred to as fertilization; in lay terms, it is more commonly known as “conception.” After the point of fertilization, the fused product of the female and male gamete is referred to as a zygote or fertilized egg. The fusion of male and female gametes usually occurs following the act of sexual intercourse, resulting in spontaneous pregnancy. However, the advent of artificial insemination and in vitro fertilisation have made achieving pregnancy possible without engaging in sexual intercourse. This approach may be undertaken as a voluntary choice or due to infertility.
The process of fertilization occurs in several steps, and the interruption of any of them can lead to failure. Through fertilization, the egg is activated to begin its developmental process, and the haploid nuclei of the two gametes come together to form the genome of a new diploid organism.
At the beginning of the process, the sperm undergoes a series of changes, as freshly ejaculated sperm is unable or poorly able to fertilize. The sperm must undergo capacitation in the female’s reproductive tract over several hours, which increases its motility and destabilizes its membrane, preparing it for the acrosome reaction, the enzymatic penetration of the egg’s tough membrane. The sperm and the egg cell, which has been released from one of the female’s two ovaries, unite in one of the two fallopian tubes. The fertilized egg, known as a zygote, then moves toward the uterus, a journey that can take up to a week to complete. Cell division begins approximately 24 to 36 hours after the male and female cells unite. Cell division continues at a rapid rate and the cells then develop into what is known as a blastocyst. When the blastocyst arrives at the uterus it attaches to the uterine wall, a process known as implantation.
The mass of cells, now known as an embryo, begins the embryonic stage which continues until cell differentiation is almost complete at eight weeks. Structures important to the support of the embryo develop, including the placenta and umbilical cord. During this time, cells begin to differentiate into the various body systems. The basic outlines of the organ, body, and nervous systems are established. By the end of the embryonic stage, the beginnings of features such as fingers, eyes, mouth, and ears become visible.
Once cell differentiation is mostly complete, the embryo enters the final stage and becomes known as a fetus. The early body systems and structures that were established in the embryonic stage continue to develop. Sex organs begin to appear during the third month of gestation. The fetus continues to grow in both weight and length, although the majority of the physical growth occurs in the last weeks of pregnancy.
Healthcare professionals name three different dates as the start of pregnancy:
- the first day of the woman’s last normal menstrual period,
- the date of conception (about two weeks before her next expected menstrual period), and
- the date of implantation (about one week after conception).
Since these are spread over a significant period of time, the duration of pregnancy necessarily depends on the date selected as the starting point chosen.
The most common system used among healthcare professionals is Naegele’s rule, which was developed in the early 19th century. This calculates the expected due date from the first day of the last normal menstrual period (LMP or LNMP) regardless of factors known to make this inaccurate, such as a shorter or longer menstrual cycle length. Pregnancy most commonly lasts for 40 weeks according to this LNMP-based method, assuming that the woman has a predictable menstrual cycle length of close to 28 days and conceives on the 14th day of that cycle, and a birth between 37 and 42 weeks LNMP is considered full-term. Other, more accurate algorithms take into account a variety of other variables, such as whether this is the first or subsequent child (i.e., pregnant woman is a primipara or a multipara, respectively), ethnicity, parental age, length of menstrual cycle, and menstrual regularity), but these are rarely used by healthcare professionals.
There is a standard deviation of 8–9 days surrounding due dates calculated with even the most accurate methods. This means that fewer than 5 percent of births occur at exactly 40 weeks; 50 percent of births are within a week of this duration, and about 80 percent are within 2 weeks. It is much more useful and accurate, therefore, to consider a range of due dates, rather than one specific day, with some online due date calculators providing this information.
Pregnancy is considered “at term” when gestation attains 37 complete weeks but is less than 42 (between 259 and 294 days since LMP). Events before completion of 37 weeks (259 days) are considered preterm; from week 42 (294 days) events are considered postterm. When a pregnancy exceeds 42 weeks (294 days), the risk of complications for both the woman and the fetus increases significantly. Therefore, in an otherwise uncomplicated pregnancy, obstetricians usually prefer to induce labor at some stage between 41 and 42 weeks.
Birth before 39 weeks, even if considered “at term”, increases the risk of complications and premature death, from factors including under-developed lungs, infection due to under-developed immune system, problems feeding due to under-developed brain, and jaundice from under-developed liver. Some hospitals in the United States have noted a significant increase in neonatal intensive care unit patients when women schedule deliveries for convenience and are taking steps to reduce induction for non-medical reasons. Complications from Caesarean section are more common than for live births.
Accurate dating of pregnancy is important, because it is used in calculating the results of various prenatal tests, (for example, in the triple test). A decision may be made to induce labor if a fetus is perceived to be overdue. Furthermore, if LMP and ultrasound dating predict different respective due dates, with the latter being later, this might signify slowed fetal growth and therefore require closer review.
Childbirth is the process whereby an infant is born. It is considered to be the beginning of the infant’s life, and age is defined relative to this event in most cultures.
A woman is considered to be in labor 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 labors, while others find that concentrating on the birth helps to quicken labor 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 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.
The postnatal period begins immediately after the birth of a child and then extends for about six weeks. During this period, the mother’s body begins the return to pre-pregnancy conditions that includes changes in hormone levels and uterus size.
Diagnosis of Pregnancy
The beginning of pregnancy may be detected in a number of different ways, either by a pregnant woman without medical testing, or by using medical tests with or without the assistance of a medical professional.
Most pregnant women experience a number of symptoms, which can signify pregnancy. The symptoms can include nausea and vomiting, excessive tiredness and fatigue, cravings for certain foods that are not normally sought out, and frequent urination particularly during the night.
A number of early medical signs are associated with pregnancy. These signs typically appear, if at all, within the first few weeks after conception. Although not all of these signs are universally present, nor are all of them diagnostic by themselves, taken together they make a presumptive diagnosis of pregnancy. These signs include the presence of human chorionic gonadotropin (“hCG”) in the blood and urine, missed menstrual period, implantation bleeding that occurs at implantation of the embryo in the uterus during the third or fourth week after last menstrual period, increased basal body temperature sustained for over 2 weeks after ovulation, Chadwick’s sign (darkening of the cervix, vagina, and vulva), Goodell’s sign (softening of the vaginal portion of the cervix), Hegar’s sign (softening of the uterus isthmus), and pigmentation of linea alba – Linea nigra, (darkening of the skin in a midline of the abdomen, caused by hyperpigmentation resulting from hormonal changes, usually appearing around the middle of pregnancy). Breast tenderness is common during the first trimester, and is more common in women who are pregnant at a young age.
Pregnancy detection can be accomplished using one or more various pregnancy tests, which detect hormones generated by the newly formed placenta. Clinical blood and urine tests can detect pregnancy 12 days after implantation. Blood pregnancy tests are more accurate than urine tests. Home pregnancy tests are urine tests, and normally cannot detect a pregnancy until at least 12 to 15 days after fertilization. A quantitative blood test can determine approximately the date the embryo was conceived.
Despite all the signs, some women may not realize they are pregnant until they are quite far along in their pregnancy. In some cases, a few women have not been aware of their pregnancy until they begin labor. This can be caused by many factors, including irregular periods (quite common in teenagers), certain medications (not related to conceiving children), and obese women who disregard their weight gain. Others may be in denial of their situation.
An early obstetric ultrasonography can determine the age of the pregnancy fairly accurately. In practice, medical professionals typically express the age of a pregnancy (i.e., an “age” for an embryo) in terms of “menstrual date” based on the first day of a woman’s last menstrual period, as the woman reports it. Unless a woman’s recent sexual activity has been limited, she has been charting her cycles, or the conception is the result of some types of fertility treatment (such as IUI or IVF), the exact date of fertilization is unknown. Without symptoms such as morning sickness, often the only visible sign of a pregnancy is an interruption of the woman’s normal monthly menstruation cycle, (i.e., a “late period”). Hence, the “menstrual date” is simply a common educated estimate for the age of a fetus, which is an average of 2 weeks later than the first day of the woman’s last menstrual period. The term “conception date” may sometimes be used when that date is more certain, though even medical professionals can be imprecise with their use of the two distinct terms. The due date can be calculated by using Naegele’s rule. The expected date of delivery may also be calculated from sonogram measurement of the fetus. This method is slightly more accurate than methods based on LMP. Additional obstetric diagnostic techniques can estimate the health and presence or absence of congenital diseases at an early stage.
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