An embryo is a human being in the earliest stage of development. Embryo is a medical term that defines a certain period of time in the development in the womb.

The only way a doctor or nurse can predict when a baby will be born is by knowing the date of the LMP or last menstrual period. Of course the size of the baby can be measured with ultrasound. The date of the LMP is assigned the landmark so to speak and the clinical staff count back 14 days prior to menstruation and make that time 0, then count forward  to term at 40 weeks [9.2 calendar months or ten lunar months from the first day of the LMP]. The measurements of a baby in the womb are usually crown to rump and crown-heel.


When fertilization occurs the oocyte,[female egg, ovum] is penetrated by the male sperm but you should know that does not always require the full sex act to happen. The cell becomes two and then four and then eight quickly and it is called a morula at that point. By day seven after fertilization it is a blastocyst a ball of cells with the life in them, that take in nourishment and divide constantly. The blastocyst has a programmed in set of instructions from the DNA code on the chromosones, so the blastocyst seeks out the warm, soft, blood-enriched lining of the woman's uterus and beds down so to speak to get nourishment from the mother. The endometrium is the place where the blastocyst embeds. By this time a cavity for the amniotic fluid exists. By day 13, the chorionic villi are present and branch out into the endometrium sort of like tiny fingers, and the yolk sac is present.Sometime between day 13 and day 19, the neural plate and folds for the brain appear. By day 20-23, the head and "tail' [lower body] appears, and the cloacal [beginnings of the anus] and hindgut appear. By day 24-27, the human form is distinct, and now is called an "embryo". The neural groove closes and the prmary brain vesicles form.The optic [eye] vesicles are present and the brain bends at the midbrain flexure. Limb 'buds' [beginnings] are present. The beginnings of the liver, pancreas, lungs, thyroid, tubules of kidneys, and heart appear.  The two heart tubes become one at the midline and the heart starts beating.

Between days 27-29 four branchial [gill like] arches develop and the ear canals are present. The lens of the eyes are forming. The leg buds appear. The heart bulges from the body at this point in time. By day 28-30 the eye infolds so that the outer portion of it is sheathed [called invagination] .The bronchi of the lungs are present.The arms and legs have lengthened. The kidneys are more developed. By day 31-32 the cerebral hemispheres [halves] are enlarging and the corpora striata are present. The lens of the eyes are formed. The nostrils appear. The arm buds have become arm-shoulder-hand, and the legs are lengthening. The gut [bowel] forms the primary loop and the ileocecal junction is identifiable.

By day 32-34 after fertilization, [about one month after fertilization]  the face has a nose. The eyes have a dark tinge from the early beginning of pigment of  the retina. Ear lobes appear. The hand has developed into separate palm and fingers [called carpal and digits] . The legs are now thighs-leg-feet distinctly.  A gut mesentery [membrane] is present.

By day 34-36, the head is larger. The trunk of the body is straighter. The nostrils are closer together adn the nasofrontal grooves are present. The ear lobes are more developed and the external auditory meatus [opening to the ear] is present. In the heart, the aortic trunk and pulmonary branches separate and there is a complete separation between the right and left atrioventricular canals. [atria=from the atrium, upper chambers of heart, and ventricular =the lower chambers, ventricles that pump].  The appendix is present, the dorsal and ventral pancreas have fused but the ducts of the pancreas remain separate. The midgut forms the umbilical cord.