It takes three to tango:
the baby!

Dept. of Obs & Gynae
Clinical School
University of Cambridge

26 Mar 2021

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What is gestation?

  • Gestation is the period of time between conception and birth when a baby grows and develops inside the mother’s womb.
  • A normal gestation lasts 37 to 42 weeks.
  • Gestational age is measured (in weeks) from the first day of mum’s last menstrual cycle.
  • During weeks 1 and 2, a woman is not yet pregnant (her body is preparing for a pregnancy).

Fertilisation (or conception)

  • At the end of week 2, an egg is released from an ovary. During intercourse, sperm enters the vagina, then travels through the uterus into the fallopian tubes.
  • Usually, a single sperm enters the egg (fertilization).
  • The combined sperm and egg (zygote) contains all the genetic information (DNA) needed to form a baby (half from mother’s egg, half from father’s sperm).

First trimester
(3-12 weeks)

So, you’re pregnant - congratulations!
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Week 3

  • The zygote multiplies into a ball of cells (blastocyst) while it travels down the fallopian tube.
  • Once the blastocyst reaches the uterus, it buries itself into the womb (implantation).

What are twin babies?

  • Identical twins happen when one fertilised embryo splits into two.
  • They have same sex, same genes and look very alike.
  • Non-identical twins happen when two eggs are fertilised by two sperm at the same time.
  • They do not have same genes or appearance and may be of different sex.

Week 4-6

  • Week 4: the ball of cells is now officially a human embryo.
    • A pregnant woman is now able to get a positive result on a home pregnancy test.
    • The baby is the size of a poppy seed.
  • Week 5: the circulatory system is beginning to form, and the tiny “heart” will start to beat this week.
    • The baby is the size of a sesame seed.
  • Week 6: the kidney, intestines and brain are beginning to develop.
    • The baby has a tail and is the size of a lentil.

Week 7-12

  • Week 7: the arms and legs are starting to form.
    • The baby is the size of a blueberry.
  • Week 8: the lungs are starting to develop.
    • The baby is the size of a kidney bean.
  • Weeks 9–12: all baby’s organs are in place.
  • From week 10 until birth, the baby is called a fetus.

Second trimester
(13-27 weeks)

The fetus begins to look more like a baby.
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Week 13-17

  • Week 13: baby is beginning to make urine and release it into the amniotic sac as amniotic fluid.
  • Week 14: baby’s sex is becoming apparent.
  • Week 17: baby’s toenails develop.

Week 18-25

  • Week 18: baby begins to hear.
  • Week 23: fingerprints and footprints form.
  • Week 25: baby responds to sounds.
  • By end of second trimester, the baby is 23 cm long from crown to rump and weigh nearly 2 pounds (820 grams).

Can I find out the sex of my baby?

  • You usually can during your second routine ultrasound scan (18 to 21 weeks of pregnancy).
  • The sonographer will not be able to be 100% certain about your baby’s sex.
  • You can also find out the sex of your baby using genetic tests.

Third trimester (28-40 weeks)

The baby will gain more weight, and prepare for delivery.
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Week 28-32

  • Week 28: baby’s eyes partially open.
  • Week 29: baby kicks and stretches.
  • Week 30: baby’s hair grows
  • Week 32: baby practices breathing

Week 33-40

  • Week 33: baby detects light
  • Week 40: due date arrives.
  • An average baby has a crown to rump length of around 36 cm and a weigh of 7 1/2 pounds (3,400 grams). It’s normal to give birth before or after your due date.

References

Quiz

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