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Resources Centre / Twins Tips Introduction / Twins Tips / Twin Pregnancy Week by Week Development Guide
Twin Pregnancy Week by Week Development Guide

When you're expecting twins, it's important to understand how your twins will develop in the womb from the moment of conception right through to the double birth day! Our Week by Week Twin Pregnancy Calendar will guide you through the early days of your multiple pregnancy from the 1st Trimester, into your 2nd Trimester and finally to the birth of your twins in your 3rd Trimester.

1st Trimester
Conception-5 weeks

Your babies are conceived by week 2 and even though you won't be aware of this, there is a lot happening inside you already! By week 3, your babies are visible to the naked eye, about the size of a pinhead! By 4 weeks, they will show up on ultrasound & although you cant see it, the amniotic fluid and embryos are inside, your babies are developing at a rapid rate.

During this time, your babies brains, bladders, livers, kidneys, reproductive tracts, spine, extremities and heart are beginning to develop. They are 2-3mm long, smaller than a grain of rice. By week 5 your babies central nervous system, muscles and bones are in the early stages of development.

6-8 Weeks

During weeks 6 and 7 your babies grow from 4mm long to 13mm, their spinal column is in place and their eyes, nose & mouth are forming. The umbilical cords attached to their placentas will provide the nourishment for your babies. Cardiac activity (heartbeats) can be seen by ultrasound which you may not be aware of until your first scan. By week 8 your babies are around 18mm long, the size of a coffee bean. Their eyelids are beginning to form, the tip of the nose is distinct and their back is elongating. Your babies length is measured from the top of their heads to their bottoms, known as crown-rump length. You may now be feeling a bit queasy and tired, gaining weight, and your breasts may feel tender.

9-10 weeks

Each baby is now a foetus, your babies are forming their organs and are beginning to move their arms and legs. The eyelids are tightly closed and the face is looking human. By week 10, your babies are 6cm, the kidneys begin to function and the heart is pumping blood around their bodies. Your doctor or midwife may plan, or you may wish, to have a caesarean section but do not feel pressured into a decision at this stage - just ensure that you are aware of all your options but remain flexible as a lot can happen between now and the birth.

11-12 weeks

Your babies are now formed and growing fast, weighing about 45g. Their lungs are still developing and are very immature, they will fully develop towards the end of the third trimester. Your babies can suck, swallow and move freely in your womb and their fingernails are beginning to form. During your 1st ultrasound scan the sonographer will confirm the number of babies its twins or more!

Theyll check how many amniotic sacs there are and see if they share a placenta. If so your pregnancy will be closely monitored for complications such as Twin to Twin Transfusion Syndrome (TTTS), Do not be alarmed, TTTS only affects a tiny amount of identical twin pregnancies. All your babies body structure is now in place and they will continue to grow at the singleton rate until week 26.

2nd Trimester
13-15 weeks

From now on the development is mostly in size. Your babies are practising breathing and their eyebrows and eyelashes are appearing. The foetal bones are growing rapidly and the limbs are lengthening. Each now weighs around 7oz and theyre around the size of a pear. About this time the placenta begins to take over the production of hormones and hopefully you will soon see an end to morning sickness.

By week 14 it is possible to tell the gender of your babies, although this is not always easy with two or more in there! You may already have felt them moving, it feels like butterflies and is an exciting time for everyone.

16-19 weeks

By sixteen weeks, the developing babies can swallow, the kidneys begin to operate and your babies now produce their own amniotic fluid. Your babies are beginning to show control over their environment! You are now entering a period of renewed energy and should be enjoying a hearty appetite. Your sleep may be disturbed by constant visits to the loo, increased dreaming and heartburn among other reasons. Find a comfortable position and try to get as much rest as possible. It is hard work carrying multiples and you need to be fair to your body and let it rest often.

Although they cannot see or hear yet, researchers believe that the babies have knowledge of each other at this point. Each foetus has the appearance of a very little baby and their movements are becoming more coordinated. Most of the development is now complete, however the babies still need to grow and their systems still need to mature.

20-23 weeks

You're over half way now and each baby is a 7.5 inch person! This is a magical month as most mums feel the foetal movements from now on. You may well be starting to feel the strain now as your babies grow, so ensure you eat well and keep your fluids up. The skin on your belly will be stretched and may be itchy. By week 22, your babies head hair is growing. The skin is translucent, pink-red in colour and wrinkled! By week 23 they can distinguish light from dark and their hearing is developing. They are in constant contact and nudge each other for space.

3rd Trimester
24-28 weeks

Week 24 is an important milestone your babies are now well along their incredible journey. Each baby is around 12 inches long and weighs about 1lb and if born now would stand a 90% chance of survival with specialist care. Your babies start to lay down fat underneath their skin to keep them warm when they are born. There is less room now so you may not feel as strong movements. During this month your babies eyelids will become unfused and when seen on ultrasound you can see your babies opening and closing their eyes.

Each baby is already very individual and have their own sleep cycles. Babies will begin to respond to sound during this month so turn on the music and talk to your babies every day. At 28 weeks, you will be about the same size as a mother expecting a single child at 40 weeks, so experiencing backache, puffy ankles, haemorrhoids and leaky breasts is likely!

28-32 weeks

From 28 weeks onwards you may be scanned more frequently to make sure your babies are growing as expected. The sonographer will check to make sure that they are in the head down position which is needed for optimal delivery. The positions that the babies are in at this point are not likely to change before delivery.

Your appetite may decrease because your uterus is increasing & your stomach is left with limited space, but it is important to ensure that you drink plenty of fluids. Some women will deliver their babies around this time, while others will remain pregnant a while longer. Your doctor may set a date for your delivery if you need a caesarean section. Otherwise, you will wait until your waters break or you start dilating and having contractions.

33-36 weeks

This is the month that a multiple pregnancy will be so different than that of a single ton one. The babies cannot grow as much as there simply isn't the room. At 34 weeks, the positioning of the lead twin will determine the mode of delivery and for triplets or higher order multiples, its usually all over by now! You should have completed your antenatal classes by week 34.

By week 36, each baby will weigh around 5lb and they are relying on your nutrition to help them put on the final layer of fat. At this point during your pregnancy you are probably feeling a sense of being done but try to enjoy these last few weeks as - believe it or not - you will soon miss the feeling of being pregnant. Twins have a tendency to arrive a little early, with around 50% being born before week 37 and by caesarean section.

37 weeks plus

Well done, you made it! If you haven't already given birth, you may be induced (labour artificially started) by week 38. Most twins will weigh around 5-5.5lb and may stay in hospital for a few days. If they are smaller than this they have a longer stay either on the special care baby unit or maternity ward in hospital until they gain weight and/or establish feeding. Labour & birth is discussed in more detail in our Twins Tip: Twin Pregnancy - Labour & Birth.



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