Fetal development for twins: The third trimester

As you go into week 28, your babies each weigh almost a kilo, and over the next nine weeks they'll put on a lot of weight in bone mass, muscle and plenty of the fat that will help them regulate their own temperature when out of the womb. While up until now your babies have both grown at about the same rate as singleton babies, in the third trimester the growth paths will diverge, with twin babies' growth rates slowing down relative to single baby growth from around week 32 (with triplets this divergence occurs nearly a month earlier, at around 29 weeks). This is because although you will usually have a bigger bump when carrying twins, you don't have twice the amount of room in your womb, so the babies' growth will be correspondingly limited. By the time they are born, twins usually weigh around two and a quarter to three kilos (and are unlikely to weigh exactly the same as each other), so they at least double their weight in the last trimester. By contrast, single babies average three and a half kilos at full term.

At the start of this trimester all your babies' organs are working with the exception of the lungs, which don't start to function until the babies are born. The lungs are by now capable of breathing air but still need more time to mature fully and should the babies be born prematurely they would need assistance breathing at the outset. The babies' heads and bodies now have the proportions they will have at birth and the freshly opened eyes, meanwhile, boast very well formed eyelashes and eyebrows and begin to react to light.

The coming weeks will be dedicated to maturing these organs, as well as putting on fat deposits ready for birth. By 35 weeks the lungs will be the only organs yet to be fully developed and most of your babies' growth is concentrated on putting on fat, developing muscle and practising the basic bodily functions, 'breathing' amniotic fluid, swallowing fluid, digesting and urinating.

Your babies' bones are continuing to grow, strengthen and harden. But the bones of your babies' skulls aren't hardening in the same way as most of the other bones in the body, they remain quite pliable to allow flexibility in the skull for the heads' journey down the birthing canal, and won't fuse together until some time after birth.

Your babies will be continuing to exercise their limbs regularly in this trimester, but activity will become less frenetic as the babies take up more and more of the available womb space.

What's happening with mum?

By now you've probably got a good idea of what your babies' movement patterns are, and it's common for babies to be very active at times when mum is resting - or trying to rest! If fetal movement has been keeping you awake at night then take some comfort from the fact that this movement is important for the development of your babies muscles, and that over the next few weeks the movement should become less intense, but no less frequent, as the babies' room for manoeuvre shrinks rapidly.

Other discomforts however, may intensify as you put on weight. Backache, leg cramps, frequent loo trips and difficulty finding a comfortable position with your ever-growing bump may all disturb your sleep at night. You may also be kept awake by anxieties surrounding the coming birth and starting your life with twins. You're probably very tired from carrying your babies around all day and, combined with your joints loosening in preparation for the birth, the extra weight can add joint pain to your list of discomforts.

If you are affected then keep trying the pain relief methods outlined for the last trimester (warm, but not hot, baths, massage, gentle exercise, pregnancy yoga), make sure you're getting a well-balanced diet and drinking lots of fluids, and do make sure that you take every opportunity you can to rest and relax during the day.

Do tell your midwife or doctor about anything other than mild discomfort, and particularly if you have unusual or severe backache, as this could be a sign of premature labour. Other danger signs to watch out for include rapid and severe swelling of your hands and feet, oedema, which could be a sign of pre-eclampsia, and frequent Braxton Hicks contractions, even if they're not very intense.

Sometime around 32 weeks it's probably a good idea to get started with having a bag packed ready for the hospital, just in case. You've probably already decided where you're giving birth, but you still may not have decided whether you're trying for a natural birth or having a planned caesarian-section. About 50% of twins are delivered by caesarian, often planned but that still leaves 50% of twins that are born vaginally. In some circumstances a planned caesarian may be the best option, and an awkward positioning of the babies often leads to a recommendation for a section: Some doctors are more willing than others to attempt vaginal delivery if the first baby is breech, so if an attempted vaginal birth is important to you may want to look for a doctor with the relevant experience with twins.

While not necessarily impossible, home birth for twins is very rare in the UK because even when one baby is engaged well in the pelvis before birth, the second baby cannot be, so the potential for complications is higher.

Towards the end of this trimester your breasts may have started to leak a watery substance called colostrum in preparation for breastfeeding. If they haven't then don't worry, this doesn't mean that you won't be able to breastfeed, some women don't start producing colostrum until after birth.

As you go into these final weeks of your pregnancy it's quite common to feel bored and fed up of being pregnant, feelings that may be mingled with an impatience to see your babies. If so, try to remember that with every passing week in the womb your babies become better prepared for life outside it.

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