As your uterus has enlarged throughout your pregnancy, the amniotic sac has expanded to accommodate both your baby and the amniotic fluid. From now on, however, the sac will grow by simply stretching rather than forming new cells.
The amniotic sac is formed from two distinct layers or membranes, an inner "amnion" and outer "chorion." The chorion originally had a blood supply of its own, but this has now been lost. The thinner amnion is able to slide over the chorion as your baby pushes against it. Neither layer contains nerve cells: this explains why it's not painful when your membranes rupture or "water breaks." Combined, the layers are only 0.5 mm thick. Collagen fibers in each layer allow for a great deal of stretch-this is vital in these final months to avoid early rupture of the membranes. Indeed, the membranes may be so resistant to rupture that they don't break until the final stages of your labor (see When am I in labor?).
In addition to holding in the amniotic fluid and providing a barrier to possible infection through the cervix, the membranes contain substances that form prostaglandins. Prostaglandins play an important part in the initiation of labor. This is one of the reasons why labor often starts when the membranes have ruptured.
The only reliable way to discover the sex of your baby is to have a diagnostic test, such as amniocentesis or chorionic villus sampling.
Even ultrasound can be wrong. And despite the common belief that the size and shape of your belly indicate gender, these are in fact determined by your muscle tone, your baby 's position, and your weight gain.
By now, it's possible to see and feel your baby moving, often with kicks and punches. This may happen more often in the evenings, when your partner finally gets to sit down and relax. Watching your baby's movements can be a great way to bond with her, and with your partner.
Your baby will respond to your voice, to music, and may even jump at unexpected noises (see ... Doctor); it's impossible to say whether this is because these noises are enjoyed or because they're irritating.
Your baby will also hiccup sometimes (see You are 18 Weeks and 2 Days) and you have a better chance of feeling her moving while she's hiccuping than any other form of movement. This is because hiccups occur over prolonged periods of time, while kicks and punches can be fairly random.
Excerpted from Pregnancy Day by Day.
Copyright ©2009 Dorling Kindersley Limited.
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