Pregnancy & Development

Fertilization Timing (egg “good for hours; sperm “good” for hours) Oviduct Capacitation enables sperm to fertilize ovum Sperm numbers – about 50 “work” on the egg

Fertilization Diploid Zygote Sex determination Twins – Dizygotic (fraternal) – Monozygotic (identical)

Cleavage 30 hours to 4 days Mitosis Oviduct Morula

Blastocyst Stage Day 5 Uterus Rearrangement of cells – Trophoblast cells will become placenta – Inner cell mass will become fetus Implantation – day 7 or 8

Gastrulation Week 2 Ectoderm – destined to become nervous tissue and skin Mesoderm – destined to become skeleton, muscles, heart, blood vessels… Endoderm – destined to become gut & lungs

Neurulation Week 3 Embryo is 2mm long (pencil eraser size) Neural groove Neural tube Destined to become brain and spinal cord

Organogenesis Week 4 Embryo 5 mm long All major organs have begun their formation Arm & Leg buds form Thalidomide & German Measles examples of what can go wrong.

Fetus By end of third month, embryo is distinctly human All major organs are established Growth for second and third trimester

Maintenance of Endometrium Estrogen & Progesterone From corpus luteum early From placenta later Birth control pills

Human Chorionic Gonadotropin HCG Made by embryo 2-12 weeks Maintains corpus luteum through first trimester

Placenta Development begins at 4 weeks and functional by 12 weeks Covers about 1/3 of inner uterus Function – Nutrients – Waste – Estrogen & Progesterone

Placenta Maternal tissue – Decidua basalis of the endometrium Fetal tissue – Chorion which develops from the trophoblast cells – Chorionic villi contain capillary beds from umbilical arteries and vein

Fetal tissue Umbilicus Amnion Amniotic fluid

Gestation 280 days 9 full months

Parturition Mild contractions due to high levels of estrogen. Once contractions occur every 30 minutes, labor occurs Amniotic fluid released Baby’s head serves as a “wedge”

Hormones Estrogen increases which causes uterine contractions, then levels drop as parturition proceeds Progesterone levels drop allowing the uterine contractions Local prostaglandins result in contractions

Hormones Oxytocin causes strong uterine contractions and works in a positive feedback mechanism Relaxin, made by placenta, dilates cervix, relaxes symphysis pubis

Afterbirth minutes later Placenta