Pre-Natal Development

The first stage of pre-natal development is cleavage. In simple terms, cleavage is the process of the newly fertilized zygote rapidly growing because of its cells dividing. This stage continues until the zygote develops into an embryo and the next stage takes place:

Implantation: This is the process when the embryo becomes attached to the uterus wall with the side of the embryonic area lying against the endometrium. The embryo grows rapidly amongst the maternal tissues forming finger-like projections which eventually become part of the placenta.

Then Differentiation occurs: the process in which unspecialised cells undergo replication to become specialised cells which carry out their own, specialised functions.

There are two types of twins: monozygotic and dizygotic.
 Monozygotic Dizygotic
  •  They are always genetically identical
  • Originates from a single sperm and a single egg
  • One placenta
  • One chorion
  • Two amniotic sacs
  •  They are never genetically identical
  • Originates from two eggs and two sperm
  • Two placentas
  • Two chorions
  • Two amniotic sacs

Exchanges between maternal and fetal circulation:
The useful transfer between mother and fetus happens across the placenta. The substances which are transferred are: carbon dioxide, oxygen, glucose and maternal antibodies.
Harmful transfer can occur between the mother and fetus also. These substances include alcohol, nicotine, drugs (e.g. heroin and thalidomide). Thalidomide is a drug which was used to stop morning sickness, however it was discovered that it was responsible for not allowing the fetus' limbs to fully develop, and was thenceforth banned.
Placental Hormones:

Throughout gestation, the endometrial layer of the uterus wall is maintained by the hormones Oestrogen and Progesterone.
Oestrogen and Progesterone also stimulate the proliferation of the milk secreting tissues of the mammary glands. Each mammary gland is composed of lobes which secrete milk into sacs connected by ducts to the nipple.
Lactation is not possible without the hormone called Prolactin. This is produced from the anterior pituitry gland and is secreted after the birth of the baby. It triggers the stimulation for lactation (producing milk).