You are now 12 weeks pregnant, which is the start of week 13. This marks the end of the 1st trimester (or the first 3 months of pregnancy) and the beginning of the 2nd trimester (the second 3 months of pregnancy). It has now been 70 days since your baby was conceived and they measure about 7.6 cm (or 3.04 inches) and weigh approximately 30 grams (about 1 ounce). As your uterus grows upwards and out of the bones of your pelvis, the pressure is taken off your bladder and your waistline may start to thicken. Although not impossible, the threat of miscarriage now significantly subsides, as do some of the early physical signs of pregnancy for many women (such as tiredness and morning sickness).
Your baby's bones are now changing from being soft and flexible (made of cartilage) to being more hardened, as the centre of them 'ossifies'. Your baby is now capable of making creeping and climbing movements, although for most women their baby is still too small to be aware of them. Very fine whiskers of hair start to appear on your baby's upper lips and eyebrows.
The placenta is now fully functional but remains larger in size than the baby (until about 16 weeks). The placenta supplies your baby with oxygen, fats, proteins, vitamins and minerals as well as removing carbon dioxide and waste materials, by 'filtering' or 'sieving' them through the placental tissues attached to the uterus. This intricate process is called 'diffusion' and the exchange is possible because the blood vessels of the mother and baby are incredibly close together, yet remarkably always stay separate.
The placenta is not a barrier, as once thought. It does allow most substances in the mother's blood stream to cross over into the baby's blood stream. These include alcohol, nicotine, caffeine, natural remedies, medications, drugs and viruses, possibly affecting their growth and development. You may wish to read lifestyle changes.
Your baby's bowels are now able to expand and contract (called 'peristalsis') and from now on they start filling with a black-green, tar-like substance that will be their first bowel motion passed after the birth, called 'meconium'.
You may want to continue following the journey of your unborn baby's development by going to baby's development - weeks 13 to 28.
Hopson JL. Fetal Psychology. Psychology Today, 1998 (Sep-Oct). Lipson T. From conception to birth - our most important journey. 1994, Millennium books, Sydney. Nilsson L, Furunhjelm M, Ingelman-Sunberg A, Wirsen C. A child is born. Revised Ed. 1977, Faber and Faber, London. Stables D. Physiology in childbearing. 2000. Harcourt - Bailliere Tindall, London. http://anatomy.med.unsw.edu.au/cbl/embryo/wwwhuman/Stages/CStages.htm
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