Fetal development of twins: Second trimester

Early in this trimester your babies will hear their first sounds and from here on in they will listen to the sounds of your body - the rhythmic thump of your heartbeat and the gurgles of your digestion. Before the end of this trimester they will be able to hear your voice, with its changing tones, and muffled sounds from outside your body.

Your babies have been moving and kicking around in your womb for a few weeks now, developing their muscles and getting stronger all the time. The twins will be in close contact with each other in the quite limited womb space they have and will probably kick and push each other through their amniotic sacs. The amniotic sacs are very thin but they're also incredibly strong and flexible, so there's no need to worry that they will rupture as the babies jostle with each other.

Sometime over this next trimester these movements will become strong enough for you feel them in side, a development called 'quickening'. The timing of quickening varies greatly from pregnancy to pregnancy - it's usually felt between weeks sixteen and twenty, but some women may feel movement even earlier, particularly if this isn't a first pregnancy. It'll take a few weeks more before your partner will be able to feel the babies' movements by placing a hand on your tummy, but you will both be able to both feel and see movement externally before this trimester is out.

As your babies move around they practice swallowing and breathing, inhaling and exhaling amniotic fluid: By week sixteen the kidneys are already functioning and the babies urinate into the amniotic fluid. Digestion also begins a few weeks into the second trimester, and meconium, your babies' first poo, begins to collect in the colons.

As you approach the half-way point of pregnancy, week 20, your babies will have developed all of their sex organs and their gender is often detectable by ultra-sound: If you don't want to know the gender of the babies then make sure you make that clear when you go for your mid-term scan.

When you reach 24 weeks you and your babies will have reached the critical milestone of 'viability'. It's not the nicest-sounding word, but it essentially means that if the twins were to be born prematurely now, they would have a decent chance of survival in a specialist intensive care unit. From here on in the outlook for premature babies improves with every day longer they stay in the womb, not only does the survival rate improve dramatically, but the risk of long-term health impacts of premature birth starts to fall.

Another important landmark comes as this trimester draws to an end, when your babies' eyes - which have been developing steadily all the while - open and start to blink for the first time.

What's happening with mum?

The second trimester is the part of pregnancy that women usually enjoy most. If you've only just discovered that you're expecting twins at your 12-week scan then you'll probably need some time to adjust to the idea. Some women will have had an inkling that a pregnancy is multiple even if this isn't confirmed before the 12-week scan - perhaps twins run in the family and you've been feeling particularly tired or nauseous in the first trimester. But even if the news doesn't come as a complete surprise, it will give you and your partner plenty to think about in the coming weeks.

Once brought to light, twin pregnancies are routinely termed as 'high risk' pregnancies, but while the risk of many pregnancy complications and discomforts is greater with twin pregnancies, there's no need to panic. Most twin pregnancies end in the birth of two healthy babies and you won't necessarily have a more difficult pregnancy just because you're expecting twins. What the term 'high risk' does mean for most mums expecting twins is a more closely observed pregnancy with more frequent health checks and scans. You'll probably be scheduled to see your doctor every two to three weeks and in the normal course of events you can expect to have between four and six ultrasound scans - more if there are any causes for concern - which is double the usual number of scans for singleton pregnancies.

If you have been hit hard by morning sickness and tiredness in the first trimester then take some comfort that in this second trimester these discomforts will usually fade, although you may have to wait a few more weeks before they do. Continuing over-tiredness may be a sign that you're running low on iron: The extra demand for red blood cells as your volume of blood increases can also put a strain on your body's iron supplies. Your doctor can perform a simple blood test to check your iron levels if tiredness is a problem. Changes to your blood supply can also lead to the common second trimester problems of bleeding gums and nosebleeds.

With twins you will usually put on weight more rapidly than single pregnancy mums, and so need to move into maternity clothes sooner. This varies quite a bit from mum to mum, but you'll probably need maternity clothes by week sixteen.

And because you put on weight more quickly than in single pregnancies, you may also find that you experience some of the discomforts that are usually common in the third trimester a little earlier than other mums. For example, as your spine curves to carry the extra weight of your expanding uterus you may have lower backache, headaches and leg pain or leg cramps (which may also indicate that you're not getting enough magnesium in your diet). You might also find that the change in weight distribution throws you off balance slightly, so you'll be better off in flat shoes or low heels. The extra weight can also lead to problems such as varicose veins and haemorrhoids.

You may be able to relieve aches and pains to some degree through exercise - prenatal yoga, walking, swimming and stretching are all good, warm baths and massages from your partner. Good nutrition is one of the keys to helping minimise many pregnancy discomforts, so make sure you're eating well when expecting twins.

If you feel faint, light-headed or dizzy occasionally as your blood pressure falls, then make sure you rest, lie down for a few minutes if you can, and recover. This is a very normal experience in pregnancy, but if you find it happens often, or severely, then make sure you mention it to your doctor or midwife.

Oedema, or swelling of the hands or feet is common during pregnancy, particularly for mums of multiples. Drinking lots of water and resting with your feet up several times a day should help, but you should always tell your midwife about any pregnancy swelling, especially if it is sudden.

Foetal screening with twins

Screening tests for congenital abnormalities and birth defects are affected by a twin pregnancy. While the nuchal translucency scan (to detect Downs Syndrome) offered between ten and fourteen weeks is just as effective for twin pregnancies as singletons, the blood screening tests usually offered between sixteen and twenty weeks aren't thought to provide accurate results for twin pregnancies, so aren't routinely offered.

If you do decide to have a nuchal translucency test and the test result shows a raised risk of Down's Syndrome for one or other of the babies (or both with identical twins) then you'll then be faced with the difficult decision of whether to undergo an amniocentesis to know for sure whether the babies are affected. While amniocentesis can detect a wide range of chromosomal and genetic abnormalities, the procedure carries twice the risk of inducing spontaneous miscarriage for twins: 2%, as opposed to 1% in singleton pregnancies. You may also then be faced with difficult decisions about what to do if one of your babies is affected, and need to make those decisions at quite an advanced stage of pregnancy, often after the half-way mark.

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