Anatomical changes during pregnancy

Anatomical changes in embryo and fetus

Anatomically, changes due to development can be grouped into a prenatal (before birth) and postnatal (after birth) periods. However, those occurring before birth falls under the changes during gestation. Changes in these prenatal periods can further be grouped under two periods of developments, which are the first to eight week, called the embryonic period and the ninth week to birth, known as the fetal period.

Generally, development is a continuous process beginning with the formation of a zygote when an oocyte (ovum) from a female is fertilized by a sperm (spermatozoon) from a male. Cell division, cell migration, programmed cell death, differentiation, growth, and cell rearrangement transform the fertilized oocyte, into a zygote and finally into a multicellular human being.

A major change that occur to the zygote is the multiplication of cells. This occurs due to a process called cleavage. Cleavage is the series of mitotic cell divisions of the zygote to formation of early embryonic cells, two cell stage, four cell stage, until the formation of a ball or mass of cells (sixteen to thirty-two cells stage) called morula.

Each of these cells are also referred to as blastomeres and the size of the cleaving zygote remains unchanged because at each succeeding cleavage division, the blastomeres become smaller. The blastomeres change their shape and tightly align themselves against each other to form a compact ball of cells (morula). This phenomenon, compaction, is probably mediated by cell surface adhesion glycoproteins. The morula stage occurs 3 to 4 days after fertilization, just as the early embryo enters the uterus.

After the morula enters the uterus a fluid-filled cavity, the blastocele, develops inside it. This changes converts the morula into a blastocyst. The centrally located cells, the inner cell mass or embryoblast, is the embryonic part of the embryo while the outer cell mass or the trophoblast contributes to the placenta. The pre-implantaion period of embryonic development is the time between fertilization and the beginning of implantation, and the period is approximately 6 days.

From the formation of zygote at fertilization to this stage, the embryo makes its way through the fallopian tube towards to uterus where it implants. Implantation is the process by which the blastocyst attaches to the endometrium of the mother’s uterine tube, the mucous membrane or lining of the uterus, and subsequently embeds in it.

Next is the formation of a gastrula. The developing blastocyst becomes a gastrula by a process called gastrulation. During gastrulation a three layered or trilaminar embryonic disc forms. The embryonic period extends from the third to eighth weeks of development and during this period three germ layers (ectoderm, mesoderm, and endoderm) subsequently differentiate into the tissues and organs of the embryo.

The stage of neurula during the third and fourth weeks is characterised by the formation of neural tube from the neural plate. It is the first appearance of the nervous system and the next stage after the gastrula. The size of embryos is given as crown-rump length (CRL), which is measured from the vertex of the cranium (crown of head) to the rump (buttocks) on ultrasound and is used to determine the gestational age of embryo or fetus.

The fetal period begins on day 57 (from 9th week) and ends when the fetus is completely outside the mother (at birth). During the fetal period, differentiation and growth of the tissues and organs formed during the embryonic period occur. The anatomical changes are more gross; for example, the placenta and the umbilical cord clearly marks the connection between the developing fetus and the mother.

Structures constituting the genitalia (phallus, urogenital fold, labioscrotal fold and the perineum) are quite identifiable. The fetus attains a CRL of 45 – 61 mm on average at the tenth week, the face changes into an identifiable human profile with the ears, eyes, nose and mouth in the normal positions. Sex is fully distinguishable at week twelve, elaboration and growth of formed structure continue, and the fetus is viable at week thirty on the average, once all the structures of respiration, particularly the alveoli sacs are formed. During the last two months of gestation increase in weight is the most striking feature.

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