Back on the computer - children still sleeping so I get a chance. Baby has been moving a lot and giving me insomnia!
As I said in my last post - women can have a difficult time with a small baby, and others, an easier time with a large baby. A woman can have a long difficult labour with a 6/7 lb baby and then has a straightforward birth with a 9-10lb baby. This happened to my cousin who, after a long, difficult first birth to an average sized baby, gave birth to a 10.5 lb baby at home in the pool without a tear or even gas and air. Why does this happen?
The main factors are the position of the baby and the position of the woman. Birth tends to be longer, more difficult, and more painful if the baby has its back towards its mother's back (sometimes called: back to back, posterior, or OP). Most babies will turn in labour - but it can take time. It is more common in first time babies - though not known why - possibly because of tighter tummy muscles or more time spent lying or tipped back on settee, in car, or in office chair compared to second time more time spent on hands and knees and leaning forward positions (search 'optimal foetal positioning' for more info). Other positions that can mean longer or more difficult births are where the baby's head is not tucked in (more common with OP babies), where there is a hand or arm up by the head, or baby's head coming down at an angle (asynclitic).
The position of the mother makes a difference too (especially to birth where the baby is not in the ideal position). Moving about, walking, rocking your hips, being upright or leaning forward all help the baby and your body to do what it needs to do. The uterus tips forward in labour. Gravity helps the baby move down. Rocking and walking helps the baby move down and to move into the best position. It also making labour less painful as your body does not need to work harder, and because often pain in labour is there to get us to move and get into positions that help labour, we do it because it eases the pain and that is what our body wants us to do. I have heard of women doing strange movements, moving legs, or getting into positions instinctively - to relieve the pain - and that happens to be just what her body needed so it could birth the baby. Being in the warmth and support of water can help too.
Lying on your back or semi seated lengthens labour but also it reduces the size of the pelvis dramatically. Women's tailbones (cocix -sp?) are hinged (unlike men's)to enable them to move back in labour. The ligaments between the pelvic bones are softened in labour to allow them to move apart to make more room. Lying on a bed stops all this happening. Lying down also means your body has to work harder. The uterus is tipped forward as I said - but in a lying position that means tipped up. The action of the first stage of labour is to shorten and move the muscle fibres up towards the fundus (pulling the cervix open) - again, if you are lying down this is against gravity. The baby is not putting pressure on the cervix so it dilates slower, and possibly leaving a lip because of uneven pressure. The baby also has to move uphill and you will be pushing up hill.
Another effect of lying down is that it is more painful, which may cause you to want an epidural, or an epidural is the cause of you lying down. Epidurals relax all the muscles from waist down. This includes the pelvic floor. This may sound like a good thing but it is not. When the baby's head touches the pelvic floor muscles the pressure and firmness prompts it to turn its head to manoeuvre through the pelvic outlet. With an epidural the muscles are floppy and they do not support the head to move, which is why epidurals are associated with a greater need for forceps or ventouse assisted births. This is particulary significant with large babies.
Being relaxed, feeling supported and able to follow your instincts all help you to birth your baby - whatever the size. A wise obstetrican I met (whose wife had 3 home births) said it is counterproductive to talk about big babies as the best way for the baby to be born is vaginally and that is more likely to happen if the woman is not fearful.
There are two concerns that relate to big babies that are worth noting - but they do not contradict what has been said above.
Firstly - a big baby can be a result of diabetes. If the mother has had diabetes that can cause the baby to have more trouble stabilising its blood sugars once it is born. Women who have diabetes prior to pregnancy will receive additional care. There is some debate whether gestational diabetes (ie caused by pregnancy) exists and of the reliability of diagnosis and the value of diagnosis (ie how does it affect outcomes.) The concerns regarding gestational diabetes are two fold: the baby's control of its blood sugar when born, and difficulties of a big baby at birth (see above and below). It is worth noting that more than half of 'macrasomic' babies (over 4kg) are born to women who do not have diabetes.
More in part two - shoulder distocia.