Your babies are now developing fingernails and toenails, and are growing very rapidly. Your belly will probably already be quite rounded, particularly if this is not your first pregnancy. One of the ways that your LMC will use to check your babies' growth at your antenatal appointments is by measuring your fundal height. Your fundal height is the measurement from the top of your uterus to your pelvic bone. In a singleton pregnancy, the measurement usually corresponds to the number of weeks pregnant you are, for example at 30 weeks, your fundal height will be around 30cm. However in a twin pregnancy, your uterus grows much faster and will be much bigger. At 13 weeks, your fundal height may be around 20cm or so. Later in your pregnancy you can expect to measure between five and ten weeks ahead of your dates, which means that by 30 weeks your fundal height will be the equivalent of a singleton pregnancy at full term!
Your babies will now weigh around 65g each and be approximately 10cm long. Up until now, pregnancy hormones in your body have been providing them with nutrients, but early in the second trimester the placenta (or placentas) take over this function. As a result, the levels of hCG and other pregnancy hormones in your blood begin to stabilise and you may find you feel less tired and that your morning sickness begins to ease. However for a few women expecting twins, morning sickness will continue until later in the second trimester, or even until their babies are born.
If you are expecting monochorionic (identical) twins you will probably have fortnightly growth scans from around 14 weeks to make sure that your babies are growing at similar rates. It is normal for one baby to be slightly larger than the other even from very early on, but on a larger scale uneven growth can be an indicator of Twin to Twin Transfusion Sydnrome (TTTS). TTTS is only a risk when twins share a placenta, so if your twins are dichorionic (have their own separate placentas) then you won't need to have these fortnightly growth scans until later in your pregnancy.
You will continue to have monthly visits with your LMC up until around 28 weeks if you have no complications, and after that your visits will be fortnightly, and then weekly. At these visits your LMC will check your blood pressure and do a dip stick urine test to check your urine for sugar and protein. He/she will also measure your fundal height and ask about any concerns you have had over the past month. Your LMC will also listen to your babies heartbeats with a handheld doppler. If your LMC is an obstetrician he/she may even have an ultrasound machine in their clinic so you can see your babies moving around.
You are now four months pregnant, and your babies are developing rapidly. Their kidneys will be beginning to function, and their skin is also forming, although at this stage it is transparent.
Now is a good time to think about whether you would like to attend antenatal classes. In some of the main centres there are special antenatal classes available for expectant parents of multiples to help prepare you for possible complications in your pregnancy, and for the birth of your babies, as well as what to expect in the early days after you bring your babies home. Your LMC or local Multiple Birth Club will be able to give you contact details for classes in your area.
If you live in an area where there are no multiples-specific antenatal classes available, you might like to consider attending standard classes to help prepare you for the birth. Bear in mind that such classes are usually held over a six-week period, and as there is a high possibility your twins may be born prematurely you should start your antenatal classes soon to ensure you finish them before your babies are born.