Anatomy and Physiology of Pregnancy

Pregnancy begins with the fertilization of an ovum by a spermatozoon as the ovum travels toward the uterus through the fallopian tube. This beginning of new life (conception) is usually the result of sexual intercourse (also termed copulation or coitus). Terms that are used to describe pregnancy are gravid or gravida, as in a gravid (pregnant) uterus; gestation, as in the time period of fetal development; and the suffixes -gravida and -cyesis. A woman who has never been pregnant is a nulligravida, whereas one who is pregnant for the first time is a unigravida (or primigravida). A woman who has been pregnant two or more times is described as a multigravida.

Time periods are important when using the terminology of pregnancy. A normal pregnancy is approximately 38 to 40 weeks, or 9 months. This time period is divided into trimesters, three 3-month segments. Specifically, the first trimester is less than 14 weeks 0 days; the second trimester is 14 weeks 0 days to less than 28 weeks 0 days; and the third trimester is 28 weeks 0 days until delivery (Fig. 7-1). Many ICD-10-CM codes require the addition of information as to the specific trimester of the patient’s pregnancy. The estimated date of delivery (EDD) of the infant can be calculated using a formula, Naegeli’s rule, that uses the first day of a woman’s last menstrual period (LMP), subtracts 3 months, and adds 7 days to come up with a “due” date. Note that this is based on 38 weeks (266 days). Forty weeks (280 days) is the standard time period for a pregnancy.

The one-celled fertilized egg, or zygote, carrying the genome (complete set of chromosomal information) divides as it moves through the fallopian tube towards the uterus (Fig. 7-2). In the first few days after fertilization, when the zygote has become a solid ball of cells from repeated divisions, it is called a morula. It is called a morula, which is Latin for mulberry, because of its similar appearance. As it continues to develop, it moves from the fallopian tube into the uterus, and becomes implanted in the uterine wall. At this point, it is identified as a blastocyst. Upon implantation, human chorionic gonadotropin (hCG), the pregnancy hormone, is secreted by the blastocyst. The hCG stimulates the corpus luteum in the ovary to continue to produce progesterone and estrogen. The continued secretion of progesterone maintains the endometrial lining to nourish the growing zygote, while the estrogen contributes to increasing the size of the lining of the uterus and the number of blood vessels that it has.

The blastocyst has a hollow outer shell, called the trophoblast, and an inner segment called the inner cell mass. The trophoblast becomes the membranes that surround and nourish the fetus, whereas the inner mass becomes the embryo and later the fetus. Notice the connection between the meaning of the combining form troph/o and its function in the developing embryo.

Twins are the result of a modification of this process. In the case of identical (monozygotic) twins, one zygote divides to develop into two genetically identical copies of chromosomes (identical genomes). When two ova are released and fertilized by two different sperm, fraternal (dizygotic) twins develop with two different sets of chromosomes (two genomes), making them not identical, but only as similar as siblings. Although a normal zygote has one set of membranes (an outer chorionic and inner amniotic) to support it through the pregnancy, twins may have a different configuration of sacs depending on when their fertilized eggs divide. If the egg divides 3 to 4 days after it is fertilized, the monozygotic twins will develop in two sets of sacs (dichorionic/diamniotic). If the division happens 3 to 8 days after fertilization, the twins will share the same outer sac (monochorionic) but each will be in its own inner sac (diamniotic). If the split occurs from 8 to 13 days after fertilization, they share the same outer and inner sac (monochorionic/monoamniotic). Finally, if the split happens after 13 days, the twins share the same outer and inner sac (monochorionic/monoamniotic), but the division is incomplete and results in conjoined twins (Fig. 7-3).

Time is also a component in naming the stages of the developing fetus. For the first 2 weeks, the developing fertilized egg is called a zygote. From weeks 3 to 8, it is termed an embryo. For the remainder of the pregnancy, it is labeled a fetus. Connecting the time periods of pregnancy with the time periods of the developing fetus, one can see that the progression from zygote to fetus occurs in the first trimester. At the end of the first trimester, the fetus weighs about 1 ounce and is 3 inches long, and the fetal membranes that support the pregnancy are developing. During the second trimester, fetal movement may be felt (“quickening”), the reproductive organs can be seen, and the fetus begins to urinate. At this time, the fetus weighs approximately 11 ounces and is 6 inches long. The third trimester is a time period of rapid weight gain, when the fetus has the best chance to survive premature delivery. At the end of this trimester, the fetus weighs about pounds and is 18 inches long. At this time, the pregnancy is considered to be at term. ICD-10 classifies late pregnancies as post-term pregnancy (from 40 to 42 completed weeks) and prolonged pregnancy (over 42 weeks).

At the same time that the embryo is developing, extraembryonic membranes are forming to sustain the pregnancy. Two of these, the amnion and the chorion, form the inner and outer sacs that contain the embryo (Fig. 7-4). The outer sac, the chorion, forms part of the placenta, a highly vascular structure that acts as a physical communication between the mother and the embryo. The chorionic villi are small projections that extend from the outer sac to provide a maximum amount of contact with the maternal blood supply. The inner sac, the amnion and its amniotic fluid, cushion the embryo, protect it against temperature changes, and allow it to move. On the outer side, the amniotic sac is connected to the yolk sac, the allantois, and to the placenta by way of the umbilical cord. The yolk sac, attached to the developing embryo, provides a source of nutrition for the early stage of development. The allantois is an embryonic structure that assists in waste removal and gas exchange. It later develops into the placenta and umbilical cord, which share similar functions. The umbilical cord is the tissue that connects the embryo to the placenta (and hence to the mother). When the baby is delivered, the umbilical cord is cut, and the baby is then dependent on his/her own body for all physiological processes. The remaining “scar” is the umbilicus, or navel.

Documentation regarding the diagnosis of pregnancy may mention the signs of pregnancy. These are divided into three types: presumptive (subjective symptoms indicative of pregnancy, but may appear in other diagnoses as well), probable (objective signs that are recognized by an examiner that may or may not indicate a pregnancy), and positive (objective signs recognized by an examiner that are present only in pregnancy).

The terms that describe delivery and childbirth are different from those of pregnancy. Parturition is the term for the act of giving birth. Related terms are antepartum (before childbirth), intrapartum (during childbirth) and postpartum (after childbirth). A mother’s parity is the number of times that she has delivered a child. The terms nullipara, unipara/primipara, and multipara are similar to those used for pregnancy. They respectively mean: no deliveries, one delivery, and two or more deliveries. ICD-10 categorizes a young primipara as a woman whose EDD is before her 16th birthday. An elderly primipara is one whose EDD is after her 35th birthday. ICD-10 also specifies that a code be used to describe grand multiparity, a woman who has delivered five or more children. The 6-week time period immediately after delivery is termed the puerperium, derived from the Latin for child and bearing. It is the period when the mother’s reproductive system returns to its pregravid state.

Stages of labor: Like the trimesters, labor is also divided into three stages: first, second, and third (Fig. 7-5). The first stage is the longest and, again, includes three parts: an early (or latent) phase, an active phase, and a transitional phase. The early phase, the longest, is marked by dilation of the cervix to approximately 4 centimeters and a changeover from irregular to regular contractions. The active phase has the now regular contractions becoming stronger and closer together. The final phase of the first stage, the transition phase, is the time when the cervix dilates to 10 centimeters and the baby moves down into the birth canal. The second stage is the time when the actual birth takes place and the umbilical cord is cut and clamped. The third stage is the delivery of the placenta.

The combining form nat/o, meaning “birth” or “born,” is often used to describe the time period around the birth of the child. It can be used to describe the newborn infant (neonate), or descriptors about the time before, around, or after birth (prenatal/antenatal, perinatal, postnatal).

Terms that are specific to neonates are related to weights (measured in grams) and weeks (referred to as immaturity). Birth weight is measured in categories of extremely low birth weight (<500 to 999 grams) and low birth weight (1000 to 2499 grams). Immaturity is categorized as “extreme” (<24 to 27 weeks) and “other” (28 to <37 weeks). On the other end of the spectrum is high weight and long gestation. High–birth weight neonates are classified as “exceptionally large” newborns (>4500 grams) and “other heavy for gestational age” (4000 to 4499 grams). Long gestation is categorized as late newborn, not heavy for gestational age; post-term is 40 to 42 weeks; and prolonged gestation is >42 weeks.

Newborn weights are termed “light for gestational age” (LGA) and “small for gestational age” (SGA). Light for gestational age means that the baby has a weight in the lowest 10th percentile, but a length above the 10th percentile. Small for gestational age means that the baby is below the 10th percentile in both weight and length.

Although newborn weights are one indicator of the state of the health of a newborn, the Apgar score rates the physical health of the infant with a set of criteria 1 minute and 5 minutes after birth. The five criteria are conveniently summarized as Appearance, Pulse, Grimace, Activity, Respiration.

17. A woman who has been pregnant two or more times is termed a/an ________________________________.

18. Once the fertilized egg has implanted in the uterine wall it is called a/an ___________________________.

19. Fraternal twins are called _____________ twins, whereas identical twins are called _____________ twins.

20. A developing fertilized egg is called a/an ___________ in its first 2 weeks, a/an __________ in weeks 3 to 8, and a/an ____________ for the remainder of the pregnancy.

21. The inner sac that contains the embryo is called the _____________, whereas the outer sac is called the ________________________________________________________________________________________________.

22. The tissue that connects the embryo to the placenta is called the __________________________________.

23. __________________ is the term for the act of giving birth.

24. A nullipara has delivered how many babies? ______________________________________________________

25. LGA stands for _____________________ and SGA stands for _________________________________________.