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Pregnancy is the carrying of one or more embryos or foetuses by female mammals, including humans, inside their bodies. In a pregnancy, there can be multiple gestations (for example, in the case of twins, or triplets). Human pregnancy is the most studied of all mammalian pregnancies.
Childbirth usually occurs about 38 weeks from fertilisation, ie. approximately 40 weeks from the start of the last menstruation. Thus, pregnancy lasts about nine months.
The term embryo is used to describe the developing human during the initial weeks, and the term foetus is used from about two months of development until birth.
A woman who is pregnant for the first time is known as a primigravida or “gravida 1″, while a woman who has never been pregnant is known as “gravida 0″. Similarly, the terms “para 0″, “para 1″ and so on are used for the number of times a woman has given birth.
In many societies’ medical and legal definitions, human pregnancy is somewhat arbitrarily divided into three trimester periods, as a means to simplify reference to the different stages of foetal development. The first trimester period carries the highest risk of miscarriage (natural death of embryo or foetus). During the second trimester the development of the foetus can start to be monitored and diagnosed. The third trimester marks the beginning of viability, or the ability of the foetus to survive, with or without medical help, outside of the mother’s womb.
Determining the start of pregnancy and predicting date of birth
Before pregnancy begins, a female oocyte (egg) must join, by spermatozoon in a process referred to in medicine as “fertilization”, or commonly known as “conception” (though the definition of the English word “conception” is somewhat controversial). Fertilization occurs usually through the act of sexual intercourse, in which a man ejaculates inside a woman, thus releasing his sperm; however, the advent of artificial insemination has made it possible for women to become pregnant if prexisting medical conditions from either the woman or the man make fertilization through sexual intercourse difficult, or if a woman chooses to become pregnant without a male partner, for any number of reasons. Though pregnancy begins at implantation, it is often convenient to date from the first day of a woman’s Last Menstrual Period (LMP). This is used to calculate the Expected Date of Delivery (EDD).
Traditionally a human pregnancy is considered to last approximately 40 weeks (280 days) from the LMP, or 38 weeks (266 days) from the date of fertilization. The 38 weeks of gestation is 9 lunar months. In the more familiar Gregorian calendar, the 40 weeks dating from the LMP is equivalent to a little more than nine months and six days, and this forms the basis of Naegele’s rule of approximating the EDD. A pregnancy is considered to have reached term between 37 and 43 weeks from the beginning of the last menstruation. Babies born before the 37 week mark are considered premature, while babies born after the 43 week mark are considered postmature.
Though these are the averages, the actual length of pregnancy depends on various factors.
In the post-implantation phase, the blastocyst secretes a hormone named human chorionic gonadotropin which in turn, stimulates the corpus luteum in the woman’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 woman. Pregnancy tests detect the presence of human chorionic gonadotropin.

Pregnancy Symptoms

Although symptoms of pregnancy may vary from woman to woman and even pregnancy to pregnancy, there is a list of common symptoms related to the onset of pregnancy. It is possible to be pregnant and experience any combination of these symptoms, and some women will experience more than others.
Symptoms of pregnancy: delayed menstruation, swollen or tender breasts, fatigue, nausea. Other symptoms may include: lower backache, headaches, frequent urination, and food cravings. Symptoms of pregnancy do not allow for a pregnancy diagnosis because each of these symptoms has the potential to be explained by other reasons (eg. missing a period because of stress).
Pregnancy is often discovered by using a pregnancy test after a “late” menstrual period. In the post-implantation phase the blastocyst secretes a hormone named human chorionic gonadotropin which in turn, stimulates the corpus luteum in the woman’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 woman. Pregnancy tests detect the presence of human chorionic gonadotropin.

Test for pregnancy

The beginning of pregnancy may be detected in a number of ways, including various pregnancy tests which detect hormones generated by the newly-formed placenta. Clinical blood and urine tests can detect pregnancy soon after implantation, which is as early as 6-8 days after fertilization. Home pregnancy tests are personal urine tests, which normally cannot detect a pregnancy until at least 12-15 days after fertilization. Both clinical and home tests can only detect the state of pregnancy, and cannot detect its age.
The doctor will use certain terms in discussing a woman’s pregnancy.

Some of the following definitions are useful:

• Intra-uterine pregnancy: A normal pregnancy occurs when a fertilized egg is implanted in the uterus (womb) and an embryo grows.

• Embryo: The term used for the developing fertilized egg during the first 12 weeks of pregnancy.

• Fetus: The term used for the developing embryo after 12 weeks of gestation

• Beta human chorionic gonadotropin (also called beta-hCG): This hormone is secreted by the placenta and can be measured to determine the presence and progression of the pregnancy. Urine or blood can be tested for its presence, and it is the hormone measured by a home pregnancy test. A positive result means a woman is pregnant; however, this test result can stay positive for several weeks after delivering a baby or after a miscarriage.

• Trimester: The length of time of a pregnancy is divided into three sections called trimesters (about three months each). Each trimester has particular events and developmental markers. For instance, the first trimester builds the foundation of the different organ systems.

• Estimated date of delivery (EDD): The delivery date is estimated by counting forward 280 days from the first day of the woman’s last period. It is also called the estimated date of confinement (EDC).

• The woman who is pregnant and her doctor will monitor the pregnancy either to prevent certain conditions from developing or to treat those conditions early. These conditions include the following:
• High-risk pregnancy: If a woman is considered to be likely to have complications during pregnancy, the pregnancy may be termed high risk. Examples include pregnancies in women with diabetes and those with high blood pressure. Age-related complications can occur in women such as teenagers, women who are over the age of 35, or women who have been treated for infertility or with pregnancies from the use of assisted reproductive technologies.

• Ectopic pregnancy: This is a pregnancy in which the egg implants somewhere other than the uterus. This can be life threatening. Ectopic pregnancy must be diagnosed early to avoid damage to the Fallopian tubes and to prevent serious illness or death. It is also called tubal pregnancy (if the egg implants in the Fallopian tubes) or extra-uterine pregnancy.

• Cervical Incompetence or Preterm Labor: This is a condition in which the cervix begins to dilate (widen) and efface (thin) before the pregnancy has reached term. Cervical incompetence can be a cause of miscarriage and preterm birth in the second and third trimesters.

• Preeclampsia/eclampsia: Preeclampsia is a systemic disease that can affect various organ systems. Vascular effects cause the blood pressure to rise in the woman who is pregnant, changes in kidney function, swelling throughout the body, and alterations in blood chemistry and nerve reflexes. If left untreated, preeclampsia can lead to eclampsia, a serious illness that causes seizures, coma, and even death.

• Multiple Gestation (for example twins, triplets): Preterm birth is twice as likely in twin pregnancies as in singleton pregnancies. The percentage of preterm birth is even greater for triplet pregnancies and quadruplet pregnancies. Preeclampsia also occurs three to five times more frequently with multiple gestation.

Timeline of a typical pregnancy

Pregnancy is typically broken into three periods, or trimesters, each of about three months. While there are no hard and fast rules, these distinctions are useful in describing the changes that take place over time.
First trimester

In medicine, pregnancy is defined as beginning when the developing embryo becomes implanted into the endometrial lining of a woman’s uterus. In some cases where complications may have arisen, the fertilized egg might implant itself in the fallopian tubes or the cervix, causing an ectopic pregnancy. Most pregnant women do not have any specific signs or symptoms of implantation, although it is not uncommon to experience light bleeding at implantation. Some women will also experience cramping during their first trimester. This is usually of no concern unless there is spotting or bleeding as well. The outer layers of the embryo grow and form a placenta, for the purpose of receiving essential nutrients through the uterine wall, or endometrium. The umbilical cord in a newborn child consists of the remnants of the connection to the placenta. The developing embryo undergoes tremendous growth and changes during the process of embryonic and foetal development.
Morning sickness afflicts about seventy percent of all pregnant women, typically only in the first trimester. Most miscarriages occur during this period.
Second trimester

Months 4 through 6 of the pregnancy are called the second trimester. Most women feel more energized in this period, and begin to put on weight as the symptoms of morning sickness subside and eventually fade away. The first movement of the foetus, often referred to as “quickening”, can be felt, as it begins to form into a recognizable shape. This typically happens by the fourth month. The placenta is now fully functioning and the foetus is making insulin and urinating. The teeth are now formed inside the foetus’ gums and the reproductive organs can be recognized, and can distinguish the foetus as male or female.
Third trimester

Final weight gain takes place, and the foetus begins to move regularly. The mother’s belly button will sometimes “pop” out due to her growing belly. This period of her pregnancy can be uncomfortable, causing symptoms like weak bladder control and back-ache. Movement of the foetus becomes stronger and more frequent and the foetus prepares for viability outside the womb through improved brain, eye, and muscle function.
It is during this time that a baby born prematurely may survive. The use of modern medical intensive care technology has greatly increased the probability of premature babies living, and has pushed back the boundary of viability to much earlier dates than would be possible without assistance. In spite of these developments, premature birth remains a major threat to the foetus, and may result in ill-health in later life, even if the baby survives.

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