Twins form from either identical (split egg) or fraternal (two separate eggs) fertilization. Early symptoms can be stronger due to higher hCG levels. Common symptoms include fatigue, breast tenderness, nausea, and frequent urination
Identical twins form from single fertilized egg splitting. Fraternal twins result from two separate eggs fertilized by different sperm. Chances of twins depend on nationality, race, age, and infertility treatment
Implantation occurs around 4 weeks, placentas begin to form. Nausea and morning sickness typically start between 5-8 weeks. Brain and spinal cord development begins at 6 weeks. Fingers, toes, and nails appear by 12 weeks. All major organs develop by 8 weeks
Hollow sac filled with transparent fluid present in all embryos. Helps exchange gas and dispose of liquid waste. First evolved as nitrogenous waste deposit in reptiles and birds. Human allantois forms umbilical artery and connects to rectum