All those changes
Changes to the mother's body tend to start quite early. They are all hormonally driven. Some of those hormones are actually produced by the fetus. This means, even though it is completely dependent on the mother, the fetus actively influences how the mother's body is going to function, to make the environment favourable for it.
Size and weight
The pace of change for the fetus is quite impressive. While it will weigh a mere 5 g at ten weeks, it will be about 300 g at twenty weeks - that's sixty times its weight, ten weeks earlier. The rate of increase then slows down steadily and it will increase in weight only about five times in the subsequent ten weeks (to 1500 g) and only about two and a half times after that (to about 3.5-4.0 kg at forty weeks).
The rate of weight increase for the fetus differs from person to person but the general pattern is the same. In fact, the speed of the change of weight in the later part of the second trimester and the whole of the third trimester is an important indicator of the health of the fetus.
As for the length of the body, many parents are shocked to learn how tiny the fetus is in the early part. An ultrasound scan at seven weeks will show the fetus very clearly if conditions are ideal. Yet, its actual length is 1 cm from top to toe! Even at twelve weeks, when limbs and even fingers can be made out as well as fetal activity appreciated on the scan, it is actually only about 5.5 cm (two inches) long.
The second trimester starts at fourteen weeks. Nothing earth-shattering happens at this point. Pregnancy is a smooth continuum and these divisions into trimesters are
arbitrary, and only meant to facilitate care of the pregnancy. They are artificial milestones in many respects. However, for those with troublesome nausea and vomiting, this is about the time they should expect to put it all behind them. A word of caution. It s unusual (but not unknown) for nausea to persist beyond this stage, and for particularly unlucky mothers, it could carry on till the eve of labour. The reason for this remains unclear.
At the start of the second trimester, the mother is still unaware of fetal movements, even though the fetus is very active. This is to do with size. It is still too small. She will start feeling the movements from about sixteen weeks (at the earliest) if she has had a previous pregnancy. For first-timers, it doesn't happen before about eighteen weeks and in some cases not until twenty weeks.
Contentions of feeling movements at fourteen weeks are as misplaced as anxiety of lack of movements at eighteen weeks, where the scan shows normal findings.
Small and large babies
As the second trimester draws to a close at about twenty-six weeks, for normal babies in a normal pregnancy, genes and the environment start to exert their influence on the eventual size of the baby.
In the second trimester - let's say at twenty weeks - it is impossible to tell by the scan findings whether the baby will be small, average-sized or large. In fact, at this stage, fetuses of the same gestational age will be quite similar in size. Those destined to be big by their genetic inheritance will start pulling away from the pack in the third trimester. Trying to determine the gestational age accurately using an ultrasound scan in the third trimester is a futile exercise. The best you can hope for is a very rough guide.