Clinical Aspects of Embryology
Q. What is the length of gestational period?
A. The period of development inside the womb from the time of conception till birth is called gestational period.
Q. what is LMP and EDD and how is EDD calculated?
A. LMP is the 1st day of last menstruation period (LMP) prior to conception. EDD is expected day of delivery. EDD is calculated by adding 9 months ± 7 days.
Q. What is the importance of determining the gestation age of developing embryo or fetus?
A. It is important to know the know the gestational age of the embryo as its required for:
- Conducting clinical procedures like chorionic villi sampling and amniocentesis.
- Interpretation of antenatal tests:
- Fetal growth assessment
- Fetal heart rate
- Alpha-fetoproteins test
- Diagnosing congenital anomalies
Q. How can the age of embryo or fetus can be estimated?
A. Gestational age of the fetus can be estimated from/by:
- The LMP (1st day of last menstrual period.
- With the use of ultrasound
- When the somites appear it can be estimated from the number of somites ( 1st pair of somites appear on 21st day and after that about 3 pairs of somites are added each day till 42-45 somites are formed at the end of 5th week).
- between 7th to 14th week age of fetus can be estimated by measuring the
- Crown – rump length (CRL): from the vertex of the skull to the midpoint between buttocks.
- Crown -heel length (CHL): from the vertex of the skull to the heel.
- In the 2nd and 3rd trimester by measuring dimensions of various body parts:
- Circumference of head
- Biparietal diameter
- Circumference of abdomen
- Length of femur
- Foot length
Q. Which methods can be used for diagnosing the congenital anomalies?
A. Many procedures are available for diagnosing congenital anomalies. They are:
- Chorionic villi sampling: this procedure can be performed between 10 – 12 weeks of gestation. A sample of chorionic villus tissue is collected by abdominal or cervical route. The sample is used for detecting chromosomal anomalies and inborn errors of metabolism.
- Amniocentesis: This procedure is usually performed between 14 – 18 week of gestation. The procedure is not performed before 14 weeks as enough of amniotic fluid is not present and the fetus is at risk. A needle is passed through transabdominal route to the amniotic cavity of the fetus under the ultrasonic guidance and 20-30 ml amniotic fluid is obtained. Amniotic fluid can be analysed for alpha fetoproteins (AFP) and acetylcholinestrase. The fetal cells in the amniotic fluid can be used for detecting chromosomal anomalies.
- Maternal serum Screening: AFP which is synthesized by fetal liver and reaches its peak about 14 weeks of gestation, it crosses placental membrane and reaches maternal circulation. The AFP levels increase in maternal serum during 2nd trimester and decrease after 30 weeks of gestation. In case of neural defects AFP levels are higher than normal.
- Ultrasonography: This procedure is non-invasive and safe. It can be used to determine:
- Fetal age and fetal growth related problems.
- Congenital anomalies such as anencephaly, spina bifida, heart and facial defects including cleft lip and palate, omphalocoele etc.
- Amount of amniotic fluid
- Position of fetus, placenta and umbilical cord.
- Multiple pregnancies.
Q. What are teratogens and teratology?
A. Agents that can cause abnormalities in the developing embryo when the embryo is exposed to them are called teratogens. The study of congenital malformations constitutes the science of teratology.
Q. What are the common causes of congenital anomalies (teratogenesis)?
A. Following are the common causes of congenital malformations:
- Hereditary: Due to defect in a specific chromosome or gene or number of chromosomes.
- Environmental:
- Infectious agents: e.g. rubella virus, cytomegalovirus, herpes simplex virus, human immunodeficiency virus (HIV) etc.
- Drugs and chemicals: e.g. thalidomide, drugs used for epilepsy ( diphhenylhydantoin, trimethadione), tranquilizers ( diazepam), alcohol, cigarette smoking.
- Hormones: administration of synthetic estrogens or progestins.
- Malnutrition: deficiencies of vitamins, minerals such as calcium and phosphorus, certain trace elements and some amino acids.
- Increased maternal age can be a factor.