First Trimester 5 week embryo ~ 1 month = 7 mm Brain and spinal cord 4 limb buds, short tail and gill pouches 9 week Fetus ~5.5 cm Placenta, umbilical cord, amnion All organs and body parts Can move

Second Trimester increase in size and refinement Ultrasound – 18 week fetus, can show sex or abnormalities 14 week fetus ~ 6 cm 20 week fetus ~ 19 cm ~ ½ kg (1 lb) Nails, eyelashes, covered with hair

Third Trimester rapid growth Premature babies 24 weeks may survive with medical care Circulatory and respiratory systems prepare for breathing air Rotates head toward cervix At birth ave = –50 cm (20 in) – kg (6-10 lb)

Childbirth Labor – strong rhythmic contractions of the uterus controlled by hormones 1.Estrogen - Induce oxytocin receptors 2.Oxytocin – causes uterus to contract, placenta to make prostaglandins (more contractions)

Childbirth… 1.Cervix dilates (0 to ~10 cm), ave hours 2.Full dilation of cervix to birth, strong contractions, ave. 20 – 60 minutes, clamp umbilical cord 3.Delivery of the placenta (afterbirth), ~ 15 minutes after birth

Hormones after birth… Estrogen and progesterone allow uterus to return to pre-pregnancy state Prolactin – promotes milk production in mammary glands –colostrum (protein and antibodies) before milk –Milk comes in ~2-3 days after birth