Answers to Clinically Oriented Questions

Health-care professionals are expected to give intelligent answers to the questions people ask, such as, “When does the baby's heart start to beat?” “When does it move its limbs?” “When is the embryo most at risk for effects from alcohol?” For prenatal diagnosis and any medical treatment before birth, physicians—especially family doctors, obstetricians, and pediatricians—need to know how the embryo and fetus develop, and also what might cause developmental defects. Moreover, research in embryology supports the application of stem cells for the treatment of certain chronic diseases.

Physicians date pregnancies from the first day of the last normal menstrual period because this date is usually remembered by women. It is not possible to detect the precise time of ovulation (discharge of ovum) or of fertilization (when development begins). Laboratory tests and ultrasound imaging can be performed to detect when ovulation is likely to occur and when pregnancy has occurred.

Pregnant women do not menstruate, even though there may be some bleeding at the usual time of menstruation. This blood may be leaking from the intervillous space of the placenta because of partial separation of the placenta from the endometrium of the uterine wall. Because there is no shedding of endometrium, this blood is not menstrual fluid; it is maternal blood that escaped from the intervillous space of the placenta.

It depends on when she forgot to take the oral contraceptive. If it was at midcycle, ovulation may occur and pregnancy could result. Taking two doses the next day would not prevent ovulation.

Coitus interruptus refers to withdrawal of the penis from the vagina before ejaculation occurs. This method is not reliable. Often, a few sperms are expelled from the penis with the secretions of the auxiliary sex glands (e.g., seminal glands) before ejaculation occurs. One of these sperms may fertilize the oocyte.

Spermatogenesis refers to the complete process of sperm formation. Spermiogenesis is the transformation of a spermatid into a sperm. Therefore spermiogenesis is the final stage of spermatogenesis.

A copper-releasing intrauterine device (IUD) may inhibit the capacitation of sperms and their transport through the uterus to the fertilization site in the uterine tube; in this case, it would be a contraceptive device. A hormone-releasing IUD (e.g., levonorgestrel) may cause changes that alter the morphologic features of the endometrium; as a result, the blastocyst does not implant. In this case the intrauterine device could be called a “contra-implantation” device.

The ovarian and menstrual cycles typically cease between 48 and 55 years of age, with the average age being 51 years. Menopause results from the gradual cessation of gonadotropin production by the pituitary gland; however, it does not mean that the ovaries have exhausted their supply of oocytes. The risk of Down syndrome and other trisomies is increased in the children of women who are 39 years or older (see Chapter 19 , Table 19.2 ). Spermatogenesis also decreases after the age of 45 years, and the number of nonviable and abnormal sperms increases. Nevertheless, sperm production continues until old age. The risk of producing abnormal gametes is much less common in men than in women; however, older men may accumulate mutations that the child might inherit. Mutations may produce birth defects (see Chapter 19 ).

Considerable research on new contraceptive methods is being conducted, including the development of oral contraceptives for men. This research includes experimental work on hormonal and nonhormonal prevention of spermatogenesis and stimulation of immune responses to sperms. Arresting the development of millions of sperms on a continuous basis has proven much more difficult than arresting the monthly development of a single oocyte. The results of molecular approaches, such as pharmacologic antagonism of the P2X1-purinoceptor and α ₁ A-adrenoceptor, may eventually provide a safe and reversible male contraceptive.

It is not known whether polar bodies are ever fertilized; however, it has been suggested that dispermic chimeras result from the fusion of a fertilized oocyte with a fertilized polar body. Chimeras are rare individuals who are composed of a mixture of cells from two zygotes. More likely, dispermic chimeras result from the fusion of dizygotic twin zygotes early in development. Dizygotic twins are derived from two zygotes. If a polar body were fertilized and remained separate from the normal zygote, it could form an embryo.

The most common cause of spontaneous abortion during the first week is chromosomal abnormality, such as the abnormalities resulting from nondisjunction (see Chapter 2 ). Failure of the syncytiotrophoblast to produce an adequate amount of human chorionic gonadotropin to maintain the corpus luteum in the ovary could also result in early spontaneous abortion.

Mitosis is the usual process of cell reproduction that results in the formation of daughter cells of the zygote. Cleavage is the series of mitotic cell divisions of the zygote. This process results in the formation of daughter cells—blastomeres. The expressions cleavage division and mitotic division have the same meaning when referring to the dividing zygote.

The nutritional requirements of the dividing zygote are not great. The nutrients are derived mainly from the secretions of the uterine tubes.

Yes. One of the blastomeres could be removed, and a Y chromosome could be identified by fluorescence staining with quinacrine mustard, or by molecular techniques (see Chapter 7 ). This technique could be made available to couples with a family history of sex-linked genetic diseases (e.g., hemophilia, muscular dystrophy), and to women who have already given birth to a child with such a disease and are reluctant to have more children. In these cases, only female embryos developing in vitro would be transferred to the uterus.

Implantation bleeding refers to the loss of small amounts of blood from the implantation site of a blastocyst that occurs a few days after the expected time of menstruation. Women unfamiliar with this possible occurrence may misinterpret the bleeding as a light menstrual flow. In such cases, they may give the physician the wrong date for their last normal menstrual period. This blood is not menstrual fluid; it is blood from the intervillous space of the developing placenta. Blood loss could also result from the rupture of chorionic arteries, veins, or both (see Chapter 8 ).

Drugs or other agents may cause early abortion of an embryo, but they do not cause birth defects if taken during the first 2 weeks. A drug or other agent either damages all the embryonic cells, killing the embryo, or injures only a few cells, in which case the embryo recovers to develop normally.

Intrauterine devices are typically very effective at preventing pregnancy by altering sperm capacitation or motility, or by altering the morphologic features of the endometrium. However, an intrauterine device does not physically block a sperm from entering the uterine tube and fertilizing an oocyte, if one is present. Although the endometrium could be hostile to implantation, a blastocyst could develop and implant in the uterine tube (i.e., ectopic tubal pregnancy). If fertilization occurs in a woman who is using an intrauterine device, the risk of ectopic pregnancy is approximately 5%.

Abdominal pregnancies are very uncommon. In most cases, it is believed to result from a tubal ectopic pregnancy. The embryo spontaneously aborts from the ruptured uterine tube and enters the peritoneal cavity. The risk of severe maternal bleeding and fetal mortality is high in cases of abdominal pregnancy. However, if the diagnosis is made late in pregnancy and the patient (mother) is free of symptoms, the pregnancy may be allowed to continue until the viability of the fetus is ensured, at which time it would be delivered by cesarean section.

Yes, certain drugs can produce birth defects if administered during the third week (see Chapter 19 ). For instance, antineoplastic agents (chemotherapy or antitumor drugs) can produce severe skeletal and neural tube defects in the embryo, such as acrania and meroencephaly (partial absence of brain), if administered during the third week.

Yes, risks to the mother 40 years or older and the embryo are increased. The most common risks are birth defects associated with chromosomal abnormalities, such as Down syndrome and trisomy 13 (see Chapter 19 ); however, women older than 40 years may have normal children. Advanced maternal age is a predisposing factor to certain medical conditions. For example, preeclampsia, a hypertensive disorder of pregnancy characterized by increased blood pressure and edema, occurs more frequently in older pregnant women than in younger ones. Advanced maternal age is also associated with a significantly increased risk to the embryo or fetus.

By the end of the eighth week, embryos and early fetuses appear similar. The name change is arbitrarily made to indicate that a new phase of development (rapid growth and differentiation) has begun and that the most critical period of development has been completed.

There are different opinions of when an embryo becomes a human being because opinions are often affected by religious and personal views. The scientific answer is that the embryo is a human being from the time of fertilization because of its human chromosomal constitution. The zygote is the beginning of a developing human. Some people consider that the embryo becomes human only after birth.

No, it cannot. During the embryonic period, more similarities than differences exist in the external genitalia (see Chapter 13 ). It is impossible to tell by ultrasound examination whether the primordial sexual organ (genital tubercle at 5 weeks and phallus at 7 weeks) will become a penis or a clitoris. Sex differences are not clear until the early fetal period (10th–12th week). Sex chromatin patterns and chromosomal analysis (fluorescence in situ hybridization) of embryonic cells obtained during amniocentesis can show the chromosomal sex of the embryo (see Chapter 7 ).