How a Twin Pregnancy Develops Week by Week

Pregnancy with twins

A twin pregnancy week by week calendar is an educational tool for moms-to-be, allowing you to see the intricate growth and progression of your precious pair. Find out what is budding inside your belly.

Twin Pregnancy

Twin pregnancy occurs in several ways. A woman may ovulate two eggs, which are fertilized by separate sperm, or she may ovulate one egg, which is fertilized by the same sperm but suddenly splits in half. These differences define identical and fraternal twins. In the earliest stages, no definite signs or symptoms can indicate twin pregnancy. In fact, twin pregnancy week by week is very similar to that of a single baby, at least in the first weeks of pregnancy. Your first introduction to your duo will probably be via an ultrasound or some other medical procedure.

Twin Pregnancy Week by Week

  • Week 6: By the sixth week, twin embryos are visible on an ultrasound. Their major organs, such as brains, kidneys, livers, and reproductive organs are developing. In addition, their arms and legs are growing.
  • Weeks 8 through 12: These weeks represent the last month of the embryonic phase. You may be fighting fatigue and nausea. Your breasts may be tender as well. However, your twins are likely thriving, their weight and length has quadrupled since gestation. Nail beds and tiny teeth buds are forming as well as fingers, toes, eyelids, and genitals.
  • Weeks 12 to 16: Your twins have earned a new name this month--fetus. Their kidneys are functioning and fluids are flowing through their body and organs. They can swallow and release amniotic fluid (by urination) and measure approximately 6 inches by week 16.
  • Weeks 16 to 20: Your twins are now fully formed, measuring approximately 8 inches each. You may be elated to feel their energy and movements inside you. The rest of their time in the womb will allow them to plump and swell, until they reach their full birth weight.
  • Weeks 20 to 24: Your twins are now coated with downy hair and vernix, a protective layer thought to preserve baby's delicate skin. Though their skin is relatively wrinkled, they are slowly filling out as fat forms, weighing in at approximately a pound and a half a piece. They have also developed footprints and fingerprints.
  • Weeks 24 to 28: Your twin's weight climbs in these weeks, doubling by week 28. They measure approximately 15 inches and have distinct features. Due to increased pressure on your bladder, you may once again encounter frequent urination. It may also be increasingly difficult to move as your belly is cumbersome and your back aches.
  • Weeks 28 to 32: Your twins weigh in at approximately 4 pounds each and measure 16 inches. Their bones are soft and pliable, but will strengthen in the last weeks, as calcium and iron are stored.
  • Weeks 32 to 36: At nearly 5 pounds each, your twins are outgrowing their home in your womb. You may be contending with contractions (Braxton Hicks) and a plethora of aches and pains, such as heartburn and insomnia. Your twin's lungs are developing rapidly, preparing for their first breaths and wails in the world. Finger and toenails are fully grown. Your twins are fattening adequately.
  • Weeks 36 to 40: If you have managed to keep your twins contained, they will be extremely cramped in their quarters. Many women give birth by week 37, though full term twin pregnancy is generally counted at week 38. These weeks will be bursting with busyness, including frequent doctor visits and exams.


Monitoring the stages of twin pregnancy week by week gives you a glimpse into your little ones' world.

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How a Twin Pregnancy Develops Week by Week