Embryonic and Fetal Development
2 weeks: Approximately 2 weeks past a woman's menstrual period, an egg is released into the fallopian tube. After the egg and sperm meet in the fallopian tube, the fertilized egg takes 3-4 days to travel down the fallopian tube into the uterus to implant. There the blastocyst rapidly divides and develops.
3 weeks: As the blastocyst continues to implant into the lining of the uterus, the amount of amniotic fluid increases to protect and nourish the blastocyst.
4 weeks: At this time, the embryo is the size of a poppy seed. It consists of two layers, the hypoblast and epiblast, that will develop into organs over the next few weeks. The egg's yolk will also develop into red blood cells and blood vessels.
5 weeks: The baby is now the size of a poppy seed. There are now 3 layers that will become the baby's central nervous system (including the brain and spinal cord), lungs, heart, and major organs.
6 weeks: The baby is the size of a bean and has a heartbeat. The baby now has impressions where the eyes, nose, ears, and mouth will form. There are also tiny buds where the arms and legs are starting to form.
7 weeks: The baby is the size of a blueberry. This week, the baby's eyes will develop in separate functional parts, such as the iris, pupils, cornea, lens, and retina. The liver, pancreas, and bowels are further developing also.
8 weeks: The baby is the size of a kidney bean and now has fingers and toes! The baby's respiratory system and central nervous system are also undergoing significant developments this week.
Hormones During Pregnancy
During pregnancy, a woman experiences elevated levels of hormones estrogen and progesterone. Estrogen promotes uterine growth, regulates other pregnancy hormones, and promotes the development of the embryo's organs. Estrogen stimulates the milk duct system within the breast to grow and differentiate in preparation for breastfeeding. The placenta produces placental growth factor and human placental lactogen (HPL) hormones to increase the blood volume and adjust the metabolism of the mother to help nourish the fetus. Progesterone levels are also elevated during pregnancy in order to promote the growth of the alveoli and lobes within the mother's breast in preparation for breastfeeding after birth. Relaxin works in collaboration with progesterone to the relax the mother's joints and ligaments in preparation for labor and childbirth.
After Birth and Lactation
Estrogen and progesterone levels are still present but decline significantly after birth in order for lactation to occur. Oxytocin is the hormone responsible for the strong uterine contractions a woman experiences after giving birth so that the uterus can shed its lining and return to its normal size. Furthermore, oxytocin is also responsible for the muscle contractions in the alveoli of the breasts in order to squeeze milk into the milk ducts of the breasts, causing the "milk let down" that results when a baby sucks on the mother's nipple. Prolactin is a hormone that is responsible for breastmilk production and the regulation of breastmilk. During pregnancy and breastfeeding, prolactin levels are elevated in order to increase insulin resistance, regulate lipid metabolism, and increase growth factor levels.